MB-7432: Add XDCR latency stats (for the toybuild)

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).

BTW, this commit piggybacks a minior fix what changes vb replicator
status from replicating to idling before reporting the stats to parent
bucket replicator, when the work is done.

Change-Id: Id57614ea98890bb03ba3ae305e0652bdb1c921f0
Merge branch 'toy' of http://github.com/xiejunyi/ns_server into toy