MB-7432 Monitor only the last N checkpoint status

Today the XDCR checkpoint stats account the number of all successful
and failed checkpoints since the beginning of replication. By request
and feedback from customer, the numbers will go very large over time
and are not helpful. This commit will change the stats to monitor
only the last 10 checkpoints status for each bucket replicator per node.

In this commit, each bucket replicator maintains a ringbuffer with
configurable size and maintains the last N checkpoint record of all
vbucket replicators under the bucket replicator. When asked for stats
from UI, the bucket replicator will count the number of successful
and failed checkpoints.

Change-Id: I393d72456515756bd82daada78507d8f48114a45
Reviewed-on: http://review.couchbase.org/24251
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>