=all #13783 replace protobuf dependency by embedded version

- created new subproject akka-protobuf (and added COPYING and LICENSE)
- renamed com.google.protobuf -> akka.protobuf everywhere
- also added such renaming step to the results of protoc compilation in
  project/Protobuf.scala
- had to include transcriptions of Netty’s ProtobufEncoder/Decoder to
  make multi-node-testkit compile again

akka-distributed-data/src/main/scala/akka/cluster/ddata/protobuf/ReplicatedDataSerializer.scala

@@ -18,7 +18,7 @@ import akka.cluster.ddata.protobuf.msg.{ ReplicatedDataMessages ⇒ rd }
 import akka.cluster.ddata.protobuf.msg.{ ReplicatorMessages ⇒ dm }
 import akka.serialization.SerializerWithStringManifest
 import akka.serialization.BaseSerializer
-import com.google.protobuf.ByteString
+import akka.protobuf.ByteString
 import akka.util.ByteString.UTF_8
 
 /**