Crash in TURN session when DNS callback invoked after TURN session destroyed

[nanang]

This is reported to happen after an immediate reject on incoming call.

Scenario:

1. DNS callback in TURN session is being invoked, and context is switched before returning from the callback
2. other thread destroys the TURN session
3. the DNS callback continues execution, e.g: TURN allocation, etc.

[nanang]

In 4551:

Fix #1685: When TURN destroy is requested in state RESOLVING, pending the destroy until resolver callback is invoked.