also: remove ClassLoader from TypedProps (and similar places)

(In revision:2ce47d6bb5e01b3862d3d9992a5b8f0e6237c903) Bye-bye ReflectiveAccess, introducing PropertyMaster, see #1750

- PropertyMaster is the only place in Akka which calls
ClassLoader.getClass (apart from kernel, which might be special)
- all PropertyMaster methods (there are only three) take a ClassManifest
of what is to be constructed, and they verify that the obtained object
is actually compatible with the required type

Other stuff:
- noticed that I had forgotten to change to ExtendedActorSystem when
constructing Extensions by ExtensionKey (damn you, reflection!)
- moved Serializer.currentSystem into JavaSerializer, because that’s the
only one needing it (it’s only used in readResolve() methods)
- Serializers are constructed now with one-arg constructor taking
ExtendedActorSystem (if that exists, otherwise no-arg as before), to
allow JavaSerializer to do its magic; possibly necessary for others as
well
- Removed all Option[ClassLoader] signatures
- made it so that the ActorSystem will try context class loader, then
the class loader which loaded the class actually calling into
ActorSystem.apply, then the loader which loaded ActorSystemImpl
- for the second of the above I added a (reflectively accessed hopefully
safe) facility for getting caller Class[_] objects by using
sun.reflect.Reflection; this is optional an defaults to None, e.g. on
Android, which means that getting the caller’s classloader is done on
a best effort basis (there’s nothing we can do because a StackTrace
does not contain actual Class[_] objects).
- refactored DurableMailbox to contain the owner val and use that
instead of declaring that in all subclasses

Branch: wip-1750-remove-ReflectiveAccess-∂π

Updating tickets (#1800, #1750, #1788, #1804, #1596, #1713, #852)