Implements a UDP port and Socks encapsulation / decapsulation.
This is for a single port. If there is demuxing for multiple
ports, it happens outside of here.
UDPSource -> SOCKSUDPUnwrapper -> ReplyTracker ( -> I2PSink in SOCKSUDPTunnel)
UDPSink <- SOCKSUDPWrapper ( <- MultiSink <- I2PSource in SOCKSUDPTunnel)
The Unwrapper passes headers to the Wrapper through a cache.
The ReplyTracker passes sinks to MultiSink through a cache.