Add more terminology

Backfill Queue, Topic, Scan Timestamp, Stable Scan
Initial Build Timestamp

Change-Id: Ia742d6f556ea2b03f1fca7aed19f353af38579d8
Reviewed-on: http://review.couchbase.org/34840
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: John Liang <john.liang@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>