Optimize get_seqs() for higher query throughput

During stale=update_after queries, we need partition seqnos to
determinte whether to trigger updater or not. The get_seqs()
involved a series of list rebuilds. By avoiding unnecessary
list rebuilds, time spend in the request_view_group_request() call
can be reduced and reduce query latency.

In a comparison test it is found that query throughput increased
from 212 qps to 300 qps.

Change-Id: I9ae57848ae39b4b42119e2cf19c947650b3fd4c4
Reviewed-on: http://review.couchbase.org/40324
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: buildbot <build@couchbase.com>