Redis memleaks and socketleaks (#1015)

Hello,

while using the `redis-statsdb` option, I found that coturn is leaking
sockets (and memory) in case of redis reconnection.

This occurs a lot to me, because in my setup I have a `coturn -> haproxy
-> redis` and if all my redis servers are down, HaProxy abruptly close
the connection to coturn and coturn reconnects periodically. After some
time I can see thousands of pending sockets (`CLOSE_WAIT`) :

```
user@server[11:32:48 UTC]:~$ sudo lsof -i | grep turn
turnserve 461797       root   15u  IPv4 12856075      0t0  TCP server:3478 (LISTEN)
turnserve 461797       root   22u  IPv4 12856081      0t0  TCP server:3478 (LISTEN)
turnserve 461797       root   23u  IPv4 12857384      0t0  UDP server:3478 
turnserve 461797       root   24u  IPv4 12857385      0t0  UDP server:3478 
turnserve 461797       root   36u  IPv4 12857390      0t0  TCP server:5766 (LISTEN)
turnserve 461797       root   43u  IPv4 12856096      0t0  TCP server:10059->haproxy-server:redis (CLOSE_WAIT)
turnserve 461797       root   46u  IPv4 12857403      0t0  TCP server:10087->haproxy-server:redis (CLOSE_WAIT)
turnserve 461797       root   48u  IPv4 12856124      0t0  TCP server:53867->haproxy-server:redis (CLOSE_WAIT)
turnserve 461797       root   50u  IPv4 12857633      0t0  TCP server:53875->haproxy-server:redis (CLOSE_WAIT)
turnserve 461797       root   51u  IPv4 12856138      0t0  TCP server:53877->haproxy-server:redis (CLOSE_WAIT)
turnserve 461797       root   54u  IPv4 12857738      0t0  TCP server:10001->haproxy-server:redis (CLOSE_WAIT)
turnserve 461797       root   55u  IPv4 12856152      0t0  TCP server:10003->haproxy-server:redis (CLOSE_WAIT)
.... (many many lines)
```


After searching and using valgrind I found 2 interesting leaks:

```
...
==460721== 32 bytes in 1 blocks are definitely lost in loss record 586 of 1,053
==460721==    at 0x483877F: malloc (vg_replace_malloc.c:307)
==460721==    by 0x1414FF: RyconninfoParse (dbd_redis.c:69)
==460721==    by 0x141B04: get_redis_async_connection (dbd_redis.c:169)
==460721==    by 0x110D7B: create_ioa_engine (ns_ioalib_engine_impl.c:407)
==460721==    by 0x12ECB0: setup_admin_thread (turn_admin_server.c:1309)
==460721==    by 0x127724: run_admin_server_thread (netengine.c:1815)
==460721==    by 0x4DA9EA6: start_thread (pthread_create.c:477)
==460721==    by 0x4EC0AEE: clone (clone.S:95)
...
==460979== 2,170 (688 direct, 1,482 indirect) bytes in 2 blocks are definitely lost in loss record 1,029 of 1,049
==460979==    at 0x483AD7B: realloc (vg_replace_malloc.c:834)
==460979==    by 0x49A1BD0: ??? (in /usr/lib/x86_64-linux-gnu/libhiredis.so.0.14)
==460979==    by 0x49A2829: redisAsyncConnect (in /usr/lib/x86_64-linux-gnu/libhiredis.so.0.14)
==460979==    by 0x13DB37: redis_reconnect (hiredis_libevent2.c:331)
==460979==    by 0x13D1A7: redisLibeventReadEvent (hiredis_libevent2.c:101)
==460979==    by 0x4D5135E: ??? (in /usr/lib/x86_64-linux-gnu/libevent_core-2.1.so.7.0.1)
==460979==    by 0x4D51A9E: event_base_loop (in /usr/lib/x86_64-linux-gnu/libevent_core-2.1.so.7.0.1)
==460979==    by 0x126D5A: run_events (netengine.c:1579)
==460979==    by 0x127272: run_general_relay_thread (netengine.c:1707)
==460979==    by 0x4DA9EA6: start_thread (pthread_create.c:477)
==460979==    by 0x4EC0AEE: clone (clone.S:95)
==460979== 
...
```

I made 1 commit for each fix.

Obviously with these fixes, I don't have anymore the leaks of thousands
of sockets (even after some time)

Thanks & hope it helps.

Thibaut

src/apps/common/hiredis_libevent2.c

@@ -325,6 +325,7 @@ static void redis_reconnect(struct redisLibeventEvents *e)
   redisAsyncContext *ac = NULL;
 
   if(e->context) {
+	  redisAsyncFree(e->context);
 	  e->context = NULL;
   }
 

src/apps/relay/dbdrivers/dbd_redis.c

@@ -62,6 +62,7 @@ static void RyconninfoFree(Ryconninfo *co) {
 		if(co->dbname) free(co->dbname);
 		if(co->password) free(co->password);
 		memset(co, 0, sizeof(Ryconninfo));
+		free(co);
 	}
 }