Remove debug line
Introduce Bunny::Concurrent::SynchronizedSortedSet

Note that it only synchronizes some operations needed by
Bunny and should not be considered a general purpose implementation.
Demonstrate that delivery tag is available with automatic ack mode
No need to spawn new threads here
Introduce useful predicates on Bunny::Consumer
Demonstrate basic.ack with automatic acknowledgement mode
Correct a bogus test

Automatic acknowledgement mode indeed won't tolerate basic.acks
Don't match on error message