gdx-udp: Library revamp as per discussed comments

Technical changes:
- UDPSocket#receiveData is no longer indefinitely blocking, returns null
when no data is available, but this behavior can be restored by setting
SO_TIMEOUT to 0
-Client and Server have been renamed to UDPClient and UDPServer
-UDPManager no longer manages alternative implementations
- Packet class no longer has methods in UDPClient and UDPServer to avoid
unneeded API bloating
- Moved a lot of UDPSocketImpl logic into UDPSocket