MB-9526: Add getRandomKey

This is used by the UI to just grab a random key in the
cache

Change-Id: I60e38d9da875656e1d7223119402f44d7c2e4b17
Reviewed-on: http://review.couchbase.org/30289
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>