Implemented Client and Server message processing

Client and Server keep incoming and outcoming messages in corresponding buffers.
Server has a pair of buffers for each connected client.
The incoming buffers are available for reading from external script; this is done by calling Client.NextMessage and ServerClients.NextMessage[i].
The contents of outcoming buffers are being sent asynchronously each tick (if there are any). If socket failed to send data, the buffer is kept for the next attempt; buffer is erased on success.