PCBC-84: Add experimental getReplica[Multi] commands

Change-Id: I7159efcfcfa105d953de9e6d9a68a5b8be33ef1b
Reviewed-on: http://review.couchbase.org/24972
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>