Bug 1083449 - Use a shared mutex instead of a per-instance mutex to protect ifc commands. r=vchang, r=chulee, a=bajaj
Bug 1080096 - Clear UDPSocketChildBase::mSocket in ReleaseIPDLReference(). r=schien, a=bajaj

This breaks a possible cycle between UDPSocketChild and UDPSocket.

The nulling out must be done before the Release, because the Release may destroy |this|.