Use asynchronous signaling to invoke pending send operations when Producer recovered

[Havret]

The current implementation of AutoRecoveringProducer is based on retries. When a connection to broker is lost, and the underlying Producer is closed, it will throw ProducerClosedException on each SendAsync or Send call. The exception is caught and after a certain, hard-coded amount of time, the operation is issued again. If the underlying producer was recovered in the meantime, the message should be sent. If however, it is still down, the exception is thrown, and the cycle starts again. This should be replaced with a solution based on signaling (hopefully asynchronous). All pending send operations should wait until the underlying producer is recovered.

Initially I was thinking about using AsyncManualResetEvent but it seems it has a nasty bug that may cause deadlocks.