Polish the API/SPI of remoting, see #2827

* Changed TransportAdapterProvider to support java impl
* Verified java impl of AbstractTransportAdapter and
  ActorTransportAdapter
* Privatized things that should not be public api
* Consistent usage of INTERNAL API marker in scaladoc
* Added some missing doc in conf
* Added missing SerialVersionUID
Merge pull request #1114 from akka/wip-2827-remote-api-patriknw

Polish API/SPI of remoting, see #2827