Refactor: move failover-table impl to .cc

Change-Id: I9e207aa30068e5d41ab358ac71d9b008ed0d4aea
Reviewed-on: http://review.couchbase.org/32354
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>