Revert "Implement replay detection."

This reverts Ib0260a0c62e3d312d2e3448a125bed64d861319e (commit a603678)

The issue we're trying to fix here is bug #1107064 - when using mirrored
queues with AMQP, acks can be lost while a master is failing over to a
slace causing the new slave to re-send messages which had previously
been acked.

The "replay detection" code applies to more than just amqp and also has
the appearance of a security measure (e.g. the use of the term 'nonce')
when clearly it serves no security purpose until we actually have
message signing.

Revert the "replay detection" approach in favour of the more targetted
amqp bugfix.

Change-Id: I8b8d15835c8b4c85cd388f5df08b60ff4c74e38d
Merge "Revert "Implement replay detection.""