MB-8516: Change seqno to revSeqno in StoredValue class

Since we will have two different sequence numbers in our
StoredValue class we should give each one a more specific
name to reduce confusion about what variables actually
represent.

Change-Id: Ie3252f3b60ca995fd032157b631e09f44a546766
Reviewed-on: http://review.couchbase.org/27187
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>