Our DTLS ENet implementation has always been quite hacky and
experimental since ENet itself lacks a proper API for DTLS.
This commit adds two more DTLS methods to our builtin ENet to allow
peer-connections lifecycle management for the DTLS server.
This means the DTLS server is now notified (by our module) when ENet
accepts or disconnects a peer, allowing it to handle timeouts and
higher-level disconnections.
Fabio Alessandrelli
d4f06d4983