This patch changes markers so that they are generated using a per-queue
side counter. A heuristic is used to mitigate a race condition. Due to the
new semantics, partial inserts are no longer possible due to collisions,
which ended up simplifying the retry logic for posting messages.
As a consequence of this patch, the last message posted no longer needs
to remain in the queue indefinitely, rendering marconi-gc unnecessary,
and so it has been removed.
Also, since the mongod GC worker runs once a minute, the queries no longer
filter out expired-but-not-yet-gc'd messages; on average, a message may
live more than 30 seconds passed it's expected lifetime, but I do not
think that this will harm or complicate any application building on top of
Marconi, practically speaking. That being said, it is worth calling out
in documentation.
Closes-Bug: #1218602
Change-Id: I34e24e7dd7c4e017c84eb5929ce37ad4c9e5266a
1512db43ec