Exclusive consumer

Here is an attempt to #301.

A couple words about it:
1) StartConsuming now returns succeed or failed status.
2) ExclusiveConsumer was introduced.  It's based on PersistentConsumer. 
3) It pulls with timeout(5s) and tries to start consuming exclusively from queue.
4) Also, ExclusiveConsumer tracks locally which queue is already in exclusive consuming
It's a first attempt, with high probability with bugs, and I want to discuss it....