Add "total", "oldest" and "newest" to queue stats

This patch introduces three new queue stats in order to expose
a better user experience via control panels. Also included in
the changes is a bug fix for calculating timestamp differences,
as well as a renaming of 'queue' to 'queue_name' for consistency
within the MongoDB driver.

Note that there are several remaining stats identified in the
associated blueprint. This may be addressed in a future
patch.

Implements: blueprint extra-queue-stats
Change-Id: I31e48a581ccdd014ec97674af0c0dd43c808c42c