Implement wait_for_reply timeout in rabbit driver

Note - the tests use timeout=0.01 because timeout=0 doesn't seem to be
working for some reason.

Change-Id: I814a3decdad5ddce0a1a2301ba2d59fa928b53a7
Remove unused IncomingMessage.done()

We appear to not have a use for this. I had originally thought we might
use this to ack messages one they've been processed and replied to, but
we actually have always acked messages as soon as they have been
deserialized and queued for dispatching.

Change-Id: I8e1fd565814f3b5e3ba0f1bc77e62ed52ff08661
Remove some FIXMEs and debug logging

These FIXMEs don't need fixing now that I think it through some more.
The reply format is specific to drivers and the 'ending' flag is part of
the existing wire protocol that we need to support even if we don't
support multicall().

Change-Id: I834f0bb01513b5318f0b365948a7d9247feb49bf