=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-remote/src/main/scala/akka/remote/serialization/DaemonMsgCreateSerializer.scala

@@ -6,7 +6,7 @@ package akka.remote.serialization
 
 import akka.serialization.{ BaseSerializer, SerializationExtension }
 import java.io.Serializable
-import com.google.protobuf.ByteString
+import akka.protobuf.ByteString
 import com.typesafe.config.{ Config, ConfigFactory }
 import akka.actor.{ Actor, ActorRef, Deploy, ExtendedActorSystem, NoScopeGiven, Props, Scope }
 import akka.remote.DaemonMsgCreate