Updated KV System Diagram And Writeup

- Changes to use Index Coordinator master/replica terminology
- Add Backfill queue to Indexers
- Show Mutation/Catch/Backfill queue per bucket
- Link John's Execution Doc from mutation flow

Change-Id: I08636fa897448da60cfdfa05a4a2d06e5a58d660
Reviewed-on: http://review.couchbase.org/34838
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-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>