MB-11903: updated stat names and descriptions for new xdcr stats

Change-Id: Ic54f819f1d137712e3b3e61cb89eef9a9d24b58f
Reviewed-on: http://review.couchbase.org/40760
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>