turn_transport: avoid leaking file descriptors

This patch fixes two issues in the shutdown function that were causing
file descriptors to remain open after they were no longer needed:
1) The TURN socket (`relay`) wasn't being destroyed properly
2) Several of the resources allocated in the TurnTransport constructor
   (such as the ioqueue and the timer_heap) weren't being cleaned up at
   all
The first issue was introduced in a recent commit (2569341f).
The second one seems to have been present for a long time (at least over
a year).

Change-Id: I82e3f96b2a08737ce0a6e3ed5aaa53959de9b1e8