Bug 981239: Use atomic ref-counting for |UnixSocketConsumer|, r=kyle

|SocketSendTask| acquires a reference to |UnixSocketConsumer| on
the main thread and releases this reference on the I/O thread;
leading to race-conditions. This patch adds

  - atomicity for ref-counting of |UnixSocketConsumer|, and
  - stricter tests in the destructor of this class.