### 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/n...)

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@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@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" ```