Implement replay detection.

Bumps the envelope revision to 2.1

Change-Id: Ib0260a0c62e3d312d2e3448a125bed64d861319e
Fast serialization of RPC envelopes for Kombu

By flattening the dictionary and turning
it into a fast-serialized string, we save
ourselves from Kombu's very slow JSON serialization
routines.

Change-Id: I64796265c7cc89a05406faabd8d7b253fe3b8acb
Merge "Implement replay detection."
Merge "Fast serialization of RPC envelopes for Kombu"