Support for concurrent message processing

[KernelPryanic]

Currently the server does handle multiple connections concurrently but it processes the messages sequentially. Requests and signals of a single client are always processed in the goroutine handling that particular client, this could turn out critical, because even if we spawn goroutines manually in the OnRequest handler for example - we'd still block other requests when waiting for the blocking one to return a response. It'd therefore make sense to move the processing of messages into separate goroutines and ideally make this feature optional by providing a server option for disabling concurrent message processing.

[romshark]

As of commit #d6105e9 messages are parsed and handled concurrently and throttled at a configured maximum number.

The server options now provide a new option: MaxConcurrentHandlers which disables throttling when set to 0 and otherwise throttles the number of concurrently executed handlers (from all connected clients) at the specified maximum number.