documentation: index_manager_design and router.

Updated StateContext in index_manager_design document.
Added connection handling details between router and indexer.

Change-Id: Iffda2156e64f8426b2fdba406b0101f24b40984c
Reviewed-on: http://review.couchbase.org/35598
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>