MB-7432: add replicate rate in outbound XDCR stats

This commit will add replication rate stats in outbound XDCR stats
section. The replication rate is measured by both number of mutations
and data (bytes) replicated per second. They are both aggregatable
stats and thus the bucket replicator on each node is responsible for
aggregating the replicate rate from each vb replicator and the then
the cluster-wide stat is computed as aggregation over all per-node
stats.

Change-Id: Ie545d846ec66cf9675e4f1aa2512da21f0a4053e
Reviewed-on: http://review.couchbase.org/24292
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Damien Katz <damien@couchbase.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>