CBIDXT-278 Send error response to scan requests during rollback

When a rollback happens, if any scan requests are waiting for a matching
atleast-timestamp snapshot to be available, fail those scan requests with
ErrIndexRollback error.

Change-Id: Ic46cc13787107ee4a5e0c33d8a2a0d3e28541704
Reviewed-on: http://review.couchbase.org/43781
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>