### Description
Setting string values via `cfg_rpc` leaves old strings in SHM.
#### Reproduction
<!-- If the issue can be reproduced, describe how it can be done. --> Set some string value to e.g. `tm.ac_extra_hdrs`: ```shell x=10 while [ $x -gt 0 ] ; do sudo kamctl rpc cfg.sets tm ac_extra_hdrs das$x >/dev/null sleep 0.001 x=$((x - 1)) done ```
#### Debugging Data
After running above snippet four times: ``` sudo kamcmd cfg.set_now_int core mem_dump_shm 1 grep "^Oct 4 15:11:4.* qm_status:.*alloc'd" /var/log/syslog | cut -d: -f6- | sort | uniq -c | sort -n | grep core/cfg/cfg 1 alloc'd from core: core/cfg/cfg_struct.c: cfg_shmize(217) 1 alloc'd from core: core/cfg/cfg_struct.c: sr_cfg_init(322) 1 alloc'd from core: core/cfg/cfg_struct.c: sr_cfg_init(334) 1 alloc'd from core: core/cfg/cfg_struct.c: sr_cfg_init(346) 1 alloc'd from core: core/cfg/cfg_struct.c: sr_cfg_init(353) 1 alloc'd from core: core/cfg/cfg_struct.c: sr_cfg_init(360) 2 alloc'd from core: core/cfg/cfg_ctx.c: cfg_register_ctx(47) 3 alloc'd from core: core/cfg/cfg_struct.c: cfg_clone_global(625) 39 alloc'd from core: core/cfg/cfg_ctx.c: cfg_set_now(559) 40 alloc'd from core: core/cfg/cfg_struct.c: cfg_child_cb_new(828) 42 alloc'd from core: core/cfg/cfg_struct.c: cfg_clone_str(130) sudo kamcmd mod.stats core shm|grep \bcfg_ cfg_clone_global(625): 1392 cfg_set_now(559): 624 cfg_clone_str(130): 384 cfg_shmize(217): 696 cfg_register_ctx(47): 64 cfg_child_cb_new(828): 2560 ``` memory values from cfg_clone_str addresses: ``` grep -B1 'Oct 4 15:11:4.*cfg_clone_str' /var/log/syslog|sed '/address=/!d' | cut -d: -f6- 55. N address=0x7f1310eab7f8 frag=0x7f1310eab7c0 size=40 used=1 56. N address=0x7f1310eab888 frag=0x7f1310eab850 size=24 used=1 62. N address=0x7f1310ec7b10 frag=0x7f1310ec7ad8 size=8 used=1 65. N address=0x7f1310ec7ca0 frag=0x7f1310ec7c68 size=8 used=1 68. N address=0x7f1310ec7e30 frag=0x7f1310ec7df8 size=8 used=1 69. N address=0x7f1310ec7ea0 frag=0x7f1310ec7e68 size=8 used=1 72. N address=0x7f1310ec8030 frag=0x7f1310ec7ff8 size=8 used=1 75. N address=0x7f1310ec81c0 frag=0x7f1310ec8188 size=8 used=1 78. N address=0x7f1310ec8350 frag=0x7f1310ec8318 size=8 used=1 81. N address=0x7f1310ec84e0 frag=0x7f1310ec84a8 size=8 used=1 84. N address=0x7f1310ec8670 frag=0x7f1310ec8638 size=8 used=1 87. N address=0x7f1310ec8800 frag=0x7f1310ec87c8 size=8 used=1 90. N address=0x7f1310ec8990 frag=0x7f1310ec8958 size=8 used=1 93. N address=0x7f1310ec8b20 frag=0x7f1310ec8ae8 size=8 used=1 96. N address=0x7f1310ec8cb0 frag=0x7f1310ec8c78 size=8 used=1 99. N address=0x7f1310ec8e40 frag=0x7f1310ec8e08 size=8 used=1 103. N address=0x7f1310ec92f0 frag=0x7f1310ec92b8 size=8 used=1 106. N address=0x7f1310ec9480 frag=0x7f1310ec9448 size=8 used=1 109. N address=0x7f1310ec9610 frag=0x7f1310ec95d8 size=8 used=1 112. N address=0x7f1310ec97a0 frag=0x7f1310ec9768 size=8 used=1 115. N address=0x7f1310ec9930 frag=0x7f1310ec98f8 size=8 used=1 118. N address=0x7f1310ec9ac0 frag=0x7f1310ec9a88 size=8 used=1 121. N address=0x7f1310ec9c50 frag=0x7f1310ec9c18 size=8 used=1 124. N address=0x7f1310ec9de0 frag=0x7f1310ec9da8 size=8 used=1 127. N address=0x7f1310ec9f70 frag=0x7f1310ec9f38 size=8 used=1 131. N address=0x7f1310eca420 frag=0x7f1310eca3e8 size=8 used=1 134. N address=0x7f1310eca5b0 frag=0x7f1310eca578 size=8 used=1 137. N address=0x7f1310eca740 frag=0x7f1310eca708 size=8 used=1 140. N address=0x7f1310eca8d0 frag=0x7f1310eca898 size=8 used=1 144. N address=0x7f1310ecad80 frag=0x7f1310ecad48 size=8 used=1 147. N address=0x7f1310ecaf10 frag=0x7f1310ecaed8 size=8 used=1 150. N address=0x7f1310ecb0a0 frag=0x7f1310ecb068 size=8 used=1 153. N address=0x7f1310ecb230 frag=0x7f1310ecb1f8 size=8 used=1 156. N address=0x7f1310ecb3c0 frag=0x7f1310ecb388 size=8 used=1 159. N address=0x7f1310ecb550 frag=0x7f1310ecb518 size=8 used=1 162. N address=0x7f1310ecb6e0 frag=0x7f1310ecb6a8 size=8 used=1 165. N address=0x7f1310ecb870 frag=0x7f1310ecb838 size=8 used=1 168. N address=0x7f1310ecba00 frag=0x7f1310ecb9c8 size=8 used=1 172. N address=0x7f1310ecbeb0 frag=0x7f1310ecbe78 size=8 used=1 175. N address=0x7f1310ecc040 frag=0x7f1310ecc008 size=8 used=1 178. N address=0x7f1310ecc1d0 frag=0x7f1310ecc198 size=8 used=1 181. N address=0x7f1310ecc360 frag=0x7f1310ecc328 size=8 used=1 0x7f1310eab7f8: "trying -- your call is important to us" 0x7f1310eab888: "Server Internal Error" 0x7f1310ec7b10: "das10" 0x7f1310ec7ca0: "das9" 0x7f1310ec7e30: "das10" 0x7f1310ec7ea0: "das10" 0x7f1310ec8030: "das9" 0x7f1310ec81c0: "das9" 0x7f1310ec8350: "das8" 0x7f1310ec84e0: "das7" 0x7f1310ec8670: "das6" 0x7f1310ec8800: "das5" 0x7f1310ec8990: "das4" 0x7f1310ec8b20: "das3" 0x7f1310ec8cb0: "das2" 0x7f1310ec8e40: "das1" 0x7f1310ec92f0: "das10" 0x7f1310ec9480: "das9" 0x7f1310ec9610: "das8" 0x7f1310ec97a0: "das7" 0x7f1310ec9930: "das6" 0x7f1310ec9ac0: "das5" 0x7f1310ec9c50: "das4" 0x7f1310ec9de0: "das3" 0x7f1310ec9f70: "das2" 0x7f1310eca420: "das1" 0x7f1310eca5b0: "das8" 0x7f1310eca740: "das7" 0x7f1310eca8d0: "das6" 0x7f1310ecad80: "das5" 0x7f1310ecaf10: "das4" 0x7f1310ecb0a0: "das3" 0x7f1310ecb230: "das2" 0x7f1310ecb3c0: "das1" 0x7f1310ecb550: "das8" 0x7f1310ecb6e0: "das7" 0x7f1310ecb870: "das6" 0x7f1310ecba00: "das5" 0x7f1310ecbeb0: "das4" 0x7f1310ecc040: "das3" 0x7f1310ecc1d0: "das2" 0x7f1310ecc360: "das1" ```
### Additional Information
* **Kamailio Version** - output of `kamailio -v`
``` /usr/sbin/kamailio -v version: kamailio 5.2.4 (x86_64/linux) flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC, DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLACKLIST, HAVE_RESOLV_RES ADAPTIVE_WAIT_LOOPS=1024, MAX_RECV_BUFFER_SIZE 262144 MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB poll method support: poll, epoll_lt, epoll_et, sigio_rt, select. id: unknown compiled with gcc 6.3.0 ```
* **Operating System**:
``` Debian 9.11 ```