UDP network channels too stream-y

[acammack-r7]

UDP channels mostly work in a client-oriented way, but lack all the datagram features. Windows meterpreter seems to support the following that mettle does not (see: https://github.com/rapid7/metasploit-payloads/blob/master/c/meterpreter/source/extensions/stdapi/server/net/socket/udp.c)

• Binding the socket to a specific port, 'LocalPort' in Rex parlance.
• Sending to arbitrary (IP, port) tuples, a la sendto
• Receiving from arbitrary senders via recvfrom

Additional minor thing: network channel reads are not written to the debug log.