(cherry picked from commit 89af8bdb90)
* remove final identifier in serializers
i* revert/deprecate ProtobufSerializer.ARRAY_OF_BYTE_ARRAY
* adding back compatible empty constructor in serializers
* make FSM.State compatible
* add back ActorPath.ElementRegex
* revert SocketOption changes and add SocketOptionV2
see a6d3704ef6
* problem filter for ActorSystem and ActorPath
* problem filter for ByteString
* problem filter for deprecated Timeout methods
* BalancingPool companion
* ask
* problem filter for ActorDSL
* event bus
* exclude hasSubscriptions
* exclude some problems in testkit
* boundAddress and addressFromSocketAddress
* Pool nrOfInstances
* PromiseActorRef
* check with 2.3.9
* migration guide note
* explicit exclude of final class problems
b30e460be7