MB-9154 wait for ns_stats_event from the correct node

- menelaus_stats_gatherer gen_server delegates stats gathering
  from the web server node to the FirstNode. this guarantees
  that we will wait for the ns_stat_event on the node
  we getting our first samples from

Backward compatibility:
- if the FirstNode has version less than 3.0 we will
  keep the current faulty behavior which will be fixed
  by the code from the following commit:
  MB-9154 ensure that stats rest api returns the same number of samples

Why it was broken:
- in case if stats are requested from the specific node
  (other than current node) menelaus_stats was waiting for
  ns_stats_event from the current node and then was retrieving
  samples from the node for which stats were requested.
  this doesn't guarantee that at least one sample that
  is later than ClientTStamp will be retrieved

- in this particular bug this led to different number of samples
  being retrieved for regular stats and for system stats.
  the conversion to JSON picks up one timestamp array out of the two
  (the one for regular stats, because it comes first)
  so for 2 timestamps we had just one entry for each system stat.
  since the client simply concatenates stat arrays the system stat
  samples were sliding against timestamps which caused
  visible graph sliding against the time grid

- also we don't have to drop the last sample if we retrieve
  stats just for one node

- this change effectively fixes the bug, but still there's
  theoretical possibility that extra sample for system stats
  will be archieved while we wait for the sample for regular
  stats. I will submit the safeguard agains such situation
  in the next commit

Change-Id: I720f8ea25d8d33a7e7136055abc246ff5b4f3bde
Reviewed-on: http://review.couchbase.org/29179
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>