[sr-dev] [kamailio/kamailio] xavp set in event_route[tm:local-request] are not used in SIP transaction (#2413)

[arkadiam]

### Description

I'm using dispatcher module like:

```
loadmodule "dispatcher.so"
modparam("dispatcher", "list_file", "/etc/kamailio/dispatcher.list")
modparam("dispatcher", "ds_probing_mode", 1)
modparam("dispatcher", "ds_ping_interval", 60)
```
With records like:
```
1 sip:sip.host.com;transport=tls  0 1 socket=tls:111.222.233.10:5061;ping_from=sip:my-domain-01.com
2 sip:sip.host.com;transport=tls  0 1 socket=tls:111.222.233.20:5061;ping_from=sip:my-domain-02.com
```

In event_route[tm:local-request] I'm modifying 'Contact' and would like to specify what client TLS profile to use via server name or server id.
```
event_route[tm:local-request] {
        if(is_method("OPTIONS")) {
               append_hf("Contact: <sip:some-host.com;transport=tls>\r\n");
               $xavp(tls=>server_name)=$fd;
               $xavp(tls[0]=>server_id)=$fd;
        }
}
```
Setting 'tls' xavp doesn't make any difference and default client TLS profile is being used.

It happens because in src/modules/tm/uac.c there is a code:
```
t_run_local_req()
...
	tm_xdata_swap(new_cell, &backup_xd, 0);
...
        /* restore original environment */
	tm_xdata_swap(new_cell, &backup_xd, 1);
```
which resets any xavp changes done in event_route[tm:local-request].

Expected behaviour: ability to use xavp in  tm:local-request route. That would allow to choose TLS client profile for dispatcher initiated requests.

### Possible Solutions

As a hack, in tm_xdata_swap() for mode=1, don't restore xavp list if it was NULL.
```
if (x->xavps_list != NULL && *x->xavps_list==NULL) {
//  .... don't restore empty list...
} else { 
        xavp_set_list(x->xavps_list);
}
```
This change allows to select required TLS profile for dispatcher requests.


### Additional Information

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

```
version: kamailio 5.3.5 (x86_64/linux) ff2f8c-dirty
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, TLS_PTHREAD_MUTEX_SHARED
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: ff2f8c -dirty
compiled on 10:19:05 Jul 28 2020 with gcc 7.5.0
```

* **Operating System**:

```
Linux xx 4.15.0-111-generic #112-Ubuntu SMP Thu Jul 9 20:32:34 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
```


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https://github.com/kamailio/kamailio/issues/2413
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