Process Stream Messages for Known Buckets Only

Add bookkeeping to Timekeeper to process stream messages for
known buckets. If the index count of a bucket goes to zero, any
subsequent stream message for that bucket will not be processed.

Change-Id: I6cb2c435f383a5fa042f61c6f98c67ecf7b9c4dc
Reviewed-on: http://review.couchbase.org/43577
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>