RCON lacked encryption, added AES-CTR encryption on RCON frames. Slightly adjusted protocol to take this into account (sending nonces, encrypted data itself, etc).
* Decoding and encoding is done into a single buffer, from raw buffers to avoid extraneous copies.
* Added base class holding all core logic for encoding, decoding, receiving and processing of the RCON protocol. This code was initially identical between all implementations of RCON, deduplicating this avoids bugs.
* Added more sophisticated error handling, stop right away when decoding for example fails.
* Added ability to have more than one active authenticated net console on the server. Controlled by cvar 'sv_rcon_maxconnections' (default 1).
* Max packet size for accepted, but not authenticated sockets is now controled by cvar 'sv_rcon_maxpacketsize' (default 1024).
