CBIDXT-83 Improve Timekeeper to track TS flush progress

Timekeeper now keeps track of TS flush for a bucket. A new stability TS
is sent only when the previous flush for the bucket is done. Otherwise,
it gets queued up.

Change-Id: Id4dbd7bf938ea87c72a3c5bcbfa9073a1cb76971
Reviewed-on: http://review.couchbase.org/38865
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>