CBES-5: response for fetch_bucket_stats call

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Change-Id: I0a97b2e0e65a8e32d4923880e7afb0516911d237
Reviewed-on: http://review.couchbase.org/23758
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>