MB-7487: Add a bulk size histogram to couchstore

Change-Id: I32bf2d93378035f57881e4b7ecbebc44bdf430a7
Reviewed-on: http://review.couchbase.org/23695
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>