Move current ActorCell field out of ActorCell to new class

The new class InternalCurrentActorCellKeeper make it possible to
update which context is used right now. This is needed for TestKit
to initially set TestActor as context when ImplicitSender is used.
TestActor is always implicit sender

Use NoImplicitSender to remove this behavior.
Merge pull request #388 from HCanber/testkit-implicit-sender

Makes TestActor implicit sender to TestKit

Mark the testclass with NoImplicitSender to remove TestActor from being implicit sender.