RAC-5743: unit test doesn't exit after upgrading mocha to 4.0.1

[PengTian0]

Paired with @mcgG
Unit test doesn't exit after upgrading mocha to 4.0.1 because the new version of mocha won't exit if there are running process.
There are 3 kinds of cases that make mocha hangs

1. amqp: try to reconnect after disconnect(connection.disconnect() fires the error handler with connection reset postwait/node-amqp#462).
   Fix: set the option "reconnect" to false in stop() to avoid the issue.
2. statsd: the socket is not closed after call statsd.stop()
   Fix: call close() in stop() to close socket.
3. stub object messenger without restore. That leads to the messenger.stop() failed to stop amqp connection.
   Fix: replace stub object with stub method of object in beforeEach. The stubbed methods will be restored in afterEach().

@mcgG @anhou @pengz1

[mcgG]

👍 +1

[anhou]

Good job!