bp:MB-7432: XDCR stats enhancement

Based on feedback from customers and our support engineers, we will
make a list of changes to improve the XDCR stats on UI. In this
commit, we rename a few current stats, in particular,

in the main bucket section:
a. Rename "XDC Dest ops/sec" to "Incoming XDCR ops/sec"
b. Rename "XDC docs to replicate" to "Outbound XDCR mutations"

in the outbound XDCR stats section:
c. Rename "mutation to replicate" to "Outbound XDCR mutations"
d. Rename "queue size" as "XDCR queue size"

Change-Id: I191193277bff9d1d34ead758977f7cc5b5c47161
Orig-Reviewed-on: http://review.couchbase.org/24189
Reviewed-on: http://review.couchbase.org/24367
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
bp: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: I205b70a3c88d93b55a1c982e7661165b360cd970
Orig-Reviewed-on: http://review.couchbase.org/24251
Reviewed-on: http://review.couchbase.org/24368
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>