MB-100: added dev docs for ns_server's usage of checkpoints

Change-Id: If16c3ace2407f060e228cb212117af4c27d37e9a
Reviewed-on: http://review.couchbase.org/26445
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
MB-8325 Adapt system_stats_collector to latest portsigar changes.

Change-Id: Ibde1162c266ceb3d9c150ff88064217f7ff71eef
Reviewed-on: http://review.couchbase.org/26434
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-8325 Send processes' stats with heartbeat.

Change-Id: Ie243e48772a039b73f47e0b73283fb7d645e19da
Reviewed-on: http://review.couchbase.org/26435
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-100 Include stats in per node diagnostics.

Change-Id: I30518cf5c2969cbb3c329ff8308d35618769b89e
Reviewed-on: http://review.couchbase.org/26436
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-100 Lower the severity of tap_estimate related error messages.

We send all the rebalance errors to the user visible log. Particularly
these error conditions don't really prevent rebalance from completing
successfully. Moreover, they are normal when mixed versions cluster is
being rebalanced. So to prevent spamming the user log we just lower
the severity of these messages.

Change-Id: Iaeb22470200b7a2cf6f96bba1428af30a43da8b4
Reviewed-on: http://review.couchbase.org/26437
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Merge remote-tracking branch 'origin/2.0.2'

* origin/2.0.2:
  MB-100 Lower the severity of tap_estimate related error messages.
  MB-100 Include stats in per node diagnostics.
  MB-8325 Send processes' stats with heartbeat.
  MB-8325 Adapt system_stats_collector to latest portsigar changes.
  MB-100: added dev docs for ns_server's usage of checkpoints

trivial merge

Change-Id: Ia1a28a4a9afe35bede5af1078513b2ede0715cc2