[ntcore] Server round robin message processing

[PeterJohnson]

Each client has an incoming queue of ClientMessage.

In the read callback:

• Parse and process only ping messages and a limited number of messages; anything else will get put into the queue and not processed
• If we queued some messages, we tell the network we stopped reading; this will result in back-pressure if we are reading too slowly. We also start an idle handle to process the queued messages.

In the idle handle callback:

• For each client, process just a few pending messages. This is performed in round-robin fashion across all clients with pending messages
• When a client's queue becomes empty, we re-enable the network read
• When all client queues are empty, we stop the idle handle (so we don't spin)

For local client processing, we use round-robin processing for most cases (including FlushLocal), but still do batch processing of all local changes for explicit network Flush() calls.