[sr-dev] [kamailio/kamailio] nat_traversal: TCP transport support (#2564)

[sergey-safarov]

### Description

Typical client device located behind NAT. If the customer use TLS or TCP transport then `nat_keepalive` cannot be used.
Details at [nat_traversal.c#L1389-L1391](https://github.com/kamailio/kamailio/blob/master/src/modules/nat_traversal/nat_traversal.c#L1389-L1391)

In this case, the TCP session may be expired on the NAT device, and future calls to the user phone not possible.

### Expected behavior
Send keep-alive messages for clients connected via TLS and TCP transport after `nat_keepalive` function call.

#### Actual observed behavior
Keep-alive messages do not send if phone connected via using TLS or TCP transport.

### Additional Information

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

```
[root at localhost kamailio]# kamailio -v
version: kamailio 5.5.0-dev3 (x86_64/linux) e9624b
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_BLOCKLIST, 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: e9624b 
compiled on 04:01:18 Nov 10 2020 with gcc 8.3.1
```

* **Operating System**:

```
[root at localhost kamailio]# cat /etc/os-release 
NAME="CentOS Linux"
VERSION="8 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="8"
PLATFORM_ID="platform:el8"
PRETTY_NAME="CentOS Linux 8 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:8"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"

CENTOS_MANTISBT_PROJECT="CentOS-8"
CENTOS_MANTISBT_PROJECT_VERSION="8"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="8"
```


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https://github.com/kamailio/kamailio/issues/2564
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