MB-100 some menelaus_stats re-factoring

- moving parsing the parameters higher in callstack
  Reason: it is quite confusing that gather_op_stats
  returns parsed parameters along with gathered stats,
  creates an impression that Step, Period and Count
  are calculated from the stats
- made some variable names more consistent, removed
  some unused parameters etc..

Change-Id: Id3f939e7333fe2ac7afbb93c205c1119e90d7385
Reviewed-on: http://review.couchbase.org/29161
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-100 get rid of the tstampParam return value

.. in stats REST API

- unnecessary and confusing

Change-Id: Ied375d5373fe76cf16a206140c7ab675a3249a5f
Reviewed-on: http://review.couchbase.org/29162
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>