Update Query System Diagram and Writeup

- Changed to use new terminology
- Linked the execution flow writeups from system diagram writeup

Change-Id: I703d9e77e18208f99a348d42acf7614d02964fa4
Reviewed-on: http://review.couchbase.org/34839
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>