As an Endpoint, I want to defer notification of received envelopes to a collection of threads to improve asynchronous performance.

1. When a message is received, add the message to an internal memory queue and queue the item for handling by another thread.

Implemented as deferring the processing of the envelope to a UserWorkItem. This should increase the throughput of the message reception.

However, looking at retlang for in-process message communication with MassTransit as an endpoint is in the near future.