Extract failover_{memcached,membase}_bucket functions.
Extract failover_{memcached,membase}_bucket functions.

And rename failover/3 to failover_bucket/3.

Change-Id: I1e9ff2de2f7e1cefeaf09c4fb2826a97f2d73524
Reviewed-on: http://review.couchbase.org/70232
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,membase/ns_server