Handle rollbacking back to zero

If a consumer needs to rollback to 0 then we return KEY_ENOENT. This
error code was not handled in the current code. When we see it we
now reset the vbucket that the stream request was sent for.

Change-Id: I56fe160e0ccdd898d15b556c5a16a443c2951312
Reviewed-on: http://review.couchbase.org/33838
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: buildbot <build@couchbase.com>