Fix race when flushing messages

The use of an AtomicBoolean and no lock meant that it was possible for a message to be queued and then never be flushed and sent to the broker:

1. On t1, a message is received and isConnected is false. The message will be queued.
2. On t2, CONNECTED is received from the broker. isConnected is set to true, the queue is drained and the queued messages are forwarded
3. On t1, the message is added to the queue

To fix this, checking that isConnected is false (step 1 above) and the queueing of a message (step 3 above) need to be performed as a unit so that the flushing of the queued messages can't be interleaved. This is achieved by synchronizing on a monitor and performing steps 1 and 3 and synchronizing on the same monitor while performing step 2.

The monitor is held while the messages are actually being forwarded to the broker. An alternative would be to drain the queue into a local variable, release the monitor, and then forward the messages. The main advantage of this alternative is that the monitor is held for less time. It also reduces the theoretical risk of deadlock by not holding the monitor while making an alien call. The downside of the alternative is that it may lead to messages being forwarded out of order. For this reason the alternative approach was rejected.