Update Scan Workflow and System diagram

Updated the diagrams to remove index manager participation
in executing scan requests.
Description has been updated accordingly.

Minor correction for arrow annotations

Change-Id: Ic6bf8028d4645a87a24ce4a4ae5cf613c865ca2a
Reviewed-on: http://review.couchbase.org/34441
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>