MB-11728 Make seqnos cache updates asynchronous

When seqno cached value has expired, we could do asynchronous updates
instead of synchronously waiting for get_seqs() call.

An async API for obtaining stats has been added in couch_upr_client and
used async method to request and update when seqnos in cache is expired.

With this change, the throughput for stale=update_after improved from
110 queries/sec to 250 queries/sec.

Change-Id: I28343bd11c7ee52a5cbf1e6326c568fcf477acc0
Reviewed-on: http://review.couchbase.org/39726
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: buildbot <build@couchbase.com>