[BP 6.0.1] Improve dcp rollback handling

Indexer's handling of dcp rollback can be improved:

1. If DCP instructs to rollback to 0, indexer ignores the disk snapshots and blindly rollbacks to 0. There may be cases where trying the stream request with disk snapshots may succeed (e.g. only vbuuid has changed). It is better to always exhaust trying dcp stream requests with all the disk snapshots before rolling back to 0.

See http://review.couchbase.org/#/c/74784/ for reference.