Prevent race condition within DNS callbacks in pjsip resolver

[sauwming]

In our DNS resolver, in order to avoid deadlock, we utilize a workaround in #1108 by releasing the lock before calling the callback.

However, this means that there can be a race condition. In sip_resolve, it is reported that the callback can be called twice when DNS A and AAAA responses are processed at the same time:

06-15 01:36:26.024   597   597 D SipTelecomPJ: 01:36:26.024     resolver.c  ...Transmitting 32 bytes to NS 0 ([...:53](http://...:53/)): DNS A query for [domain.net](http://domain.net/): Success
06-15 01:36:26.026   597   923 D SipTelecomPJ: 01:36:26.026   pjsua_core.c  TX 745 bytes Request msg SUBSCRIBE/cseq=1832 (tdta0x9061e068) to TLS [...:5061](http://...:5061/):
06-15 01:36:26.026   597   923 D SipTelecomPJ: SUBSCRIBE  ...
...

06-15 01:36:26.027   597   597 D SipTelecomPJ: 01:36:26.024  sip_resolve.c  ...DNS AAAA record resolution failed: No answer record in the DNS response (PJLIB_UTIL_EDNSNOANSWERREC)
06-15 01:36:26.027   597   597 D SipTelecomPJ: 01:36:26.027   pjsua_core.c !...TX 745 bytes Request msg SUBSCRIBE/cseq=1832 (tdta0x9061e068) to TLS [...:5061](http://...:5061/):
06-15 01:36:26.027   597   597 D SipTelecomPJ: SUBSCRIBE ...
...
06-15 01:36:26.028   597   597 D SipTelecomPJ: 01:36:26.028 evsub0x9060c86  .....Subscription state changed NULL --> SENT cause