MB-8516: Change id to bySeqno 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: I141d666361d7005a84884397188bcd1b7a53b894
Reviewed-on: http://review.couchbase.org/27188
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>