indexer/* router/* (API to construct KeyVersions message)

- API will now construct KeyVersions message instead of Mutation
  message.
- API for DropData.
- added `seqno` argument to NewStreamBegin and NewStreamEnd
  messages.
- Rebuilt test cases and benchmarks.

indexer/* (connection library for mutation stream)

- go formating.
- Instead of calling StreamServer we now call it as MutationStream
  (on the indexer side of the connection).
- Worked on John's review comment.

Change-Id: I828230672b926ab2987a7150a967b6be2ce7e8b9
Reviewed-on: http://review.couchbase.org/35604
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>