Use longpoll rather than normal for changes feed on _replicator db
Use longpoll rather than normal for changes feed on _replicator db

This helps prevent the case where changes to the _replicator db have not
fully propagated, resuting in an empty changes feed and a stuck replication

BugzID: 12759

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb