Guarantee FIFO/once-and-only-once delivery when using MongoDB

This patch modifies the way message markers are generated and
used, such that Marconi can guarantee FIFO for a single message
producer posting to a single queue (FIFO for multiple producers
is not guaranteed.) At the same time, these changes guarantee
that observer clients will recieve messages once-and-only once
by removing race conditions inherent in timestamp-based markers.

Along the way, some technical debt was also addressed, particulary
regarding style, as well as some minor optimizations.

A new service, marconi-mongo-gc, was also added, and is required
when using the MongoDB storage driver for Marconi. This service
was necessary due to the special requirement that at least the
most recent message always remain in each queue, which is an
a-priori assumption upon which the FIFO/pagination algorithm
is based, and arises from the particular constraints imposed
by MongoDB's semantics.

Note: While implementing this blueprint, many calculated tradeoffs
were made in an attempt to balance performance, risk, readability,
and maintainability. The goal was to create a resonable baseline
implementation that can be iterated upon pending comprehensive
system and performance testing. Due to the many subtleties
of solving the FIFO/once-and-only-once problem for the MongoDB
driver, future contributors should excercise extreme caution when
modifying the algorithm introduced in this patch.

Changes include:
* Align text in comments
* Add counter to queue, messages
* Markers are now converted over to using monotonic counter
* Handle DuplicateKeyError
* Return resources in body as a response to a message POST
* Added mongo driver claims tests
* Return 503 when no messages were enqueued due to marker conflict
* Added backoff sleep between retries
* Added marconi-mongo-gc service. This is a new required servi

Implements: blueprint message-pagination
Change-Id: Ifa0bb9e1bc393545adc4c804d14c6eb2df01848c
Merge "Guarantee FIFO/once-and-only-once delivery when using MongoDB"