MB-7535 Get seqs of indexable partitions synchronously

If there are update listeners or waiting clients (stale=false
query requests), ensure we get the latest seq numbers of all
the indexable partitions by asking the db set process to get
them synchronously.

Change-Id: Ic303f4ac83034e7a91f7af832d8864c1a8ff5549
Reviewed-on: http://review.couchbase.org/24026
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>