MB-8516: Rename seqno and id variables in Item class

This change makes the code more readable by changing
some old variable names to make them more descriptive
based on what they do today.

seqno -> revSeqno
id -> bySeqno

Change-Id: I6aec6693a5c46ed08f861120e17afc684f8d8802
Reviewed-on: http://review.couchbase.org/27335
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>