protobuf: stream messages

protobuf/ directory shall contain all *.proto definitions.
protobuf/Makefile contains commands to build *.go file from *.proto files.
$ make build

removed topicid field from Mutation structure.
removed bucket field from Mutation structure.
removed EndCommand from Command enumeration.
Added DropData command
Added VbConnectionMap message
Added VbucketMutation message
Mutation message can carry more than one VbucketMutation messages.

Change-Id: I5fc82befefa8d5eff12ed00caed11e91b036c99e
Reviewed-on: http://review.couchbase.org/35597
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>