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: Id11fd9deca7e24c164f4e2815722936ad29d23ea
Reviewed-on: http://review.couchbase.org/24189
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>