Implemented MongoTimeseries.delete()
Spec'd out inserting according to the various types, including working
around mongo restrictions on atomic updates. See documentation block in
insert() for problems, options and current solution.
Implemented mongo timeseries get(), requiring an index order change for
resolution.
Don't allocate a dictionary for resolution data if it's not needed in
redis series().
Implemented mongo timseries series()
Updating changelog with note about mongo support