Bug 830704: Delete UnixSocketImpl instance after SocketReceiveTasks completed [r=qdot]

The Bluetooth system internally uses UnixSocketImpl when transfering
files. When Bluetooth gets disabled during a file transfer, the IPC code
deletes any related instance of UnixSocketImpl. This can happen before all
pending SocketReceiveTasks have been processed by the main thread. The
implementation of SocketReceiveTask uses a reference to the instance of
UnixSocketImpl that has just deen disabled. This results in a segmantation
fault.

This patch fixes the problem by scheduling the delete operation for
UnixSocketImpl to be executed after any pending SocketReceiveTasks.