Design documentation.

- index_manager overview document.
- index_manager design document.
- projector overview document.
- projector design document.
- router overview document.

worked on John's comment

Worked on Deep's, John's feedback

- Drill down index topology design
- STREAM_BEGIN and STREAM_END solution for kv-rebalance

Change-Id: I8d7b67c2419df63e36cb6e053fc67595db5b2c31
Reviewed-on: http://review.couchbase.org/34688
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>