IO layer: add Java API and docs for TCP
Added Java API for UdpFF
add INTERNAL API markers and explicit return types
Java API for UdpConn
Added missing return types in UdpFFMessage
fix TcpConnection’s PendingWrite.wantsAck
Merge pull request #1132 from akka/wip-IO-Java-∂π

IO layer: Java API and docs, make Write accept NoAck aux data