Improve TS Generation logic

There can be cases when snapshot marker comes before all stream begins
have been received and if there are no further mutations, indexer
will not generate stability timestamp. This has been fixed now.

Change-Id: Ie0115ff36b5486488c5e287341eb9ab9c5d20eeb
Reviewed-on: http://review.couchbase.org/43443
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>