MB-7432: Add XDCR latency stats

This commit adds two new stats in outbound XDCR stat section,
namely, 1) meta ops latency and 2) doc ops latency, which measure
the latency of metadata operations and document update operations
in XDCR. The unit is millisecond.

Both stats are computed as the weighted average of the stats
from all vb replicator worker processes. In particular, each
vb replicator worker process will monitor two latency stats
and use the number of checked docs (replicated docs) as weight
of meta ops latency (doc ops latency, respectively). Both
the aggregated latency and its weight are passed all the way
up to vb replicator, bucket replicator, node stat collector,
and cluster-wise stats collector. The finally aggregation is
computed as the weighted average (the total aggregated latency
divided by total weights).

Change-Id: Id57614ea98890bb03ba3ae305e0652bdb1c921f0