MB-10368: Get sequence numbers via UPR

Instead of getting the sequence numbers of the partitions from
the database files (as couch_db_set does), get the sequence numbers
directly via UPR.

Change-Id: I35ed32a01beb30b61df6f399a4b13a79f00ac122
Reviewed-on: http://review.couchbase.org/34436
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>