First integration with indexer.

- Added partition key field and evaluate document for partition key.
  * Changed Evaluator and Router interface.
- Changed protobuf field names to match Go native structures by indexer.

Patchset 2:
- Changed SecondaryKey() API to Transform()

Change-Id: I35873a196e9ca648ec9e4b598223063234b8b45a
Reviewed-on: http://review.couchbase.org/38320
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>