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

akka-docs/scala/code/akka/docs/serialization/SerializationDocSpec.scala

@@ -6,7 +6,7 @@ package akka.docs.serialization
 import org.scalatest.matchers.MustMatchers
 import akka.testkit._
 //#imports
-import akka.actor.ActorSystem
+import akka.actor.{ ActorSystem, PropertyMaster }
 import akka.serialization._
 import com.typesafe.config.ConfigFactory
 
@@ -35,8 +35,7 @@ class MyOwnSerializer extends Serializer {
   // using the type hint (if any, see "includeManifest" above)
   // into the optionally provided classLoader.
   def fromBinary(bytes: Array[Byte],
-                 clazz: Option[Class[_]],
-                 classLoader: Option[ClassLoader] = None): AnyRef = {
+                 clazz: Option[Class[_]]): AnyRef = {
     // Put your code that deserializes here
     //#...
     null
@@ -147,9 +146,7 @@ class SerializationDocSpec extends AkkaSpec {
     val bytes = serializer.toBinary(original)
 
     // Turn it back into an object
-    val back = serializer.fromBinary(bytes,
-      manifest = None,
-      classLoader = None)
+    val back = serializer.fromBinary(bytes, manifest = None)
 
     // Voilá!
     back must equal(original)