create histograms for some xdcr stats
create histograms for some xdcr stats

Change-Id: I74bffe16549b2c75ca03993c9cd6b1757559dc4b
Reviewed-on: http://review.couchbase.org/30540
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Pavel Paulau <dd88eded64e90046a680e3a6c0828ceb8fe8a0e7@gmail.com>

couchbase/cbmonitor,couchbase/cbmonitor