[sr-dev] [kamailio/kamailio] EVAPI: Better ability to remove dead clients (#1880)

[weili-jiang]

### Description

If an EVAPI client drops dead and silently closes its TCP connection, EVAPI is not notified in an acceptable time frame. The default OS TCP keep-alive parameters are generally far too long to be effective, and there is no way to override these using module or runtime configuration. Moreover, any attempts to put in a workaround is thwarted by the inability to close an arbitrary EVAPI connection from say, a timer event (since there is no way to specify a connection to close outside the EVAPI context).

### Expected behavior

When an EVAPI client goes dead, the connection should be closed in the order of seconds (ideally configurable). The EVAPI should allow connections to be closed via scripting from any context.

#### Actual observed behavior

When an EVAPI client goes dead, the connection stays open (counting towards the maximum number of allowed clients). When the evapi_close is invoked from other contexts, nothing happens.

#### Debugging Data

#### Log Messages

#### SIP Traffic

### Possible Solutions

### Additional Information

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

```
version: kamailio 5.2.1 (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 GNU/Linux 9 (stretch)

```
Linux 3b83b180f7c9 4.15.0-45-generic #48-Ubuntu SMP Tue Jan 29 16:28:13 UTC 2019 x86_64 GNU/Linux
```


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https://github.com/kamailio/kamailio/issues/1880
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