MB-11387 Compute index seq nos before starting updater

The end sequence numbers upto which the updater has to index
should be precalculated before starting the updater. We update
the index one partition at a time. Currently the end sequence number
is computed just before reading from the partition upr stream.
By the time one parition stream has been read, the next partition
sequence would have advanced to a higher value and it accumulates
further for each partition. This results in stale=false queries
to wait for long time since one updater instance would take too
much time to notify all the waiters.

If a compactor process has been already started, it will have to
chase the updater and do all the work of applying lot of deltas.
This will also result in compactor never able to catchup and index
files will remain fragmented all the time.

Change-Id: Ib768fced2a7251d797812f3687159f2cc0e59663
Reviewed-on: http://review.couchbase.org/38402
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>