fixes to mutation path from UPR to indexer data-port.

- helper method Length() on KeyVersions object.
- error logging for stats.
- error logging for data-port daemon.
- bug fix in ConnectBucket(), flushBuffers(), doEvaluate(), NewProjector(),
  getBucket().
- bug fix in KVFeed data path.
- adapt scatterMutation() function to UPR interface.
- adapt vbucketRoutine to UPR interface.

- go-formatting for protobuf/index.go

mutation path breathing from UPR to indexer data-port for beer-sample bucket.
2 indexes, 1 instance per index, 1 simple key index, 1 composite key index.

    refer to tools/datapath.go

Patchset 2:
- resetEngine() outside requestFeed's loop.

Change-Id: I68c81e742b5f52aab88c1f63871e81d789b86e05
Reviewed-on: http://review.couchbase.org/38559
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>