=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-persistence/src/main/scala/akka/persistence/serialization/MessageSerializer.scala

@@ -10,7 +10,7 @@ import akka.persistence._
 import akka.persistence.fsm.PersistentFSM.StateChangeEvent
 import akka.persistence.serialization.{ MessageFormats ⇒ mf }
 import akka.serialization._
-import com.google.protobuf._
+import akka.protobuf._
 import scala.collection.immutable.VectorBuilder
 import scala.concurrent.duration
 import akka.actor.Actor