MB-100: save xdcr stats to json file

Change-Id: I74e639ddd97d9046e4cce0917d5146536c0bf3eb
Reviewed-on: http://review.couchbase.org/24442
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>