Cleanup: protobuf interfaces.

Change-Id: If8c23b3708d3b88902fbac5002abf9825a871418
Reviewed-on: http://review.couchbase.org/39234
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>