[sr-dev] [kamailio/kamailio] dispatcher algorithm 11, no more than 25 hosts in group for proper distribution (#2698)

[E1isIvan]

### Description

We tried to load share 50+ media servers using dispatching algorithm 11 and faced an unexpected behavior when most calls go to very first host in the group.

### Troubleshooting
I tried to start Kamailio like this:
sudo kamailio -f /etc/kamailio/kamailio.cfg -E -d 5 -u 995
but errors were the same when dispatcher group had more than 25 and less than 25 hosts

It was tried different combinations of dispatcher list host group configuration:
100 sip:10.60.27.123:7000 10 rweight=50

100 sip:10.60.27.123:7000 10 weight=50;rweight=50

100 sip:10.60.27.123:7000 rweight=50,maxload=80

When making 100 calls the quarter of them go to the first host in the group.

#### Reproduction
modparam("dispatcher", "list_file", "/etc/kamailio/dispatcher.list")
modparam("dispatcher", "flags", 2)
modparam("dispatcher", "ds_ping_method", "OPTIONS")
modparam("dispatcher", "ds_probing_threshold", 3)
modparam("dispatcher", "ds_inactive_threshold", 10)
modparam("dispatcher", "ds_probing_mode", 3)
modparam("dispatcher", "ds_ping_interval", 10)
modparam("dispatcher", "ds_ping_reply_codes", "501,403,404,400,200")
modparam("dispatcher", "ds_ping_from",DS_PING_FROM_PARAM)
modparam("dispatcher", "use_default", 0)

if ( ds_is_from_list("101")) {
        sl_send_reply("100","My calls");
        ds_select_dst("100", "11");
        return;
}

Dispatcher group must have more than 25 hosts, if equal or less than 25 than it is Ok:
100 sip:10.60.27.123:7000 0 10 rweight=50
100 sip:10.60.27.123:7001 0 10 rweight=50
100 sip:10.60.27.123:7002 0 10 rweight=50
100 sip:10.60.27.123:7003 0 10 rweight=50
100 sip:10.60.27.123:7004 0 10 rweight=50
100 sip:10.60.27.123:7005 0 10 rweight=50
100 sip:10.60.27.123:7006 0 10 rweight=50
100 sip:10.60.27.123:7007 0 10 rweight=50
100 sip:10.60.27.123:7008 0 10 rweight=50
100 sip:10.60.27.123:7009 0 10 rweight=50
100 sip:10.60.27.123:7010 0 10 rweight=50
100 sip:10.60.27.123:7011 0 10 rweight=50
100 sip:10.60.27.123:7012 0 10 rweight=50
100 sip:10.60.27.123:7013 0 10 rweight=50
100 sip:10.60.27.123:7014 0 10 rweight=50
100 sip:10.60.27.123:7015 0 10 rweight=50
100 sip:10.60.27.123:7016 0 10 rweight=50
100 sip:10.60.27.123:7017 0 10 rweight=50
100 sip:10.60.27.123:7018 0 10 rweight=50
100 sip:10.60.27.123:7019 0 10 rweight=50
100 sip:10.60.27.123:7020 0 10 rweight=50
100 sip:10.60.27.123:7021 0 10 rweight=50
100 sip:10.60.27.123:7022 0 10 rweight=50
100 sip:10.60.27.123:7023 0 10 rweight=50
100 sip:10.60.27.123:7024 0 10 rweight=50

To emulate call load and multiple media servers, SIPp scenarios were used.

#### Debugging Data

#### Log Messages

#### SIP Traffic

No specific traffic, regular INVITE messages having international numbers.

### Possible Solutions

### Additional Information

  * **Kamailio Version** - output of `kamailio -v`

```
kamailio -v
version: kamailio 5.4.4 (x86_64/linux) e16352
flags: USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, 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: e16352 
compiled on 15:56:46 Feb 15 2021 with gcc 4.8.5
```

* **Operating System**:

```
CentOS Linux release 7.7.1908 (Core)

Linux test-carrier-1.prd.tc5.ams.nl.kwebbl.loc 3.10.0-1062.4.1.el7.x86_64 #1 SMP Fri Oct 18 17:15:30 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
```


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https://github.com/kamailio/kamailio/issues/2698
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