stun.c: Fix ast_stun_request() erratic timeout.

If ast_stun_request() receives packets other than a STUN response then we
could conceivably never exit if we continue to receive packets with less
than three seconds between them.

* Fix poll timeout to keep track of the time when we sent the STUN
request.  We will now send a STUN request every three seconds regardless
of how many other packets we receive while waiting for a response until we
have completed three STUN request transmission cycles.

Change-Id: Ib606cb08585e06eb50877f67b8d3bd385a85c266