XDCR: allow multiple replications to be created between buckets

Currently, XDCR does not allow creating/defining multiple replications (create replication) from a source bucket to same remote cluster/bucket.  For example, after you create a replication from the travel-sample bucket, replicating just tenant_agent_00 and tenant_agent_01, you cannot also create another replication from the same travel-sample bucket, replicating tenant_agent_02 and tenant_agent_03, to the same remote cluster/bucket.  Would like that restriction to be lifted.