Jardin Server should provide a way of alerting when pending connections reach a given limit

[Aleksander Morgado]

In the unexpected and undesired situation where all worker threads in a Jardin Server get stuck in a deadlock, the server will still accept() incoming connections which will be added to the internal list of connections to be processed, and the list will grow forever, until the number of FDs for the process reaches the system-configured limit.

We should provide a new method for the Jardin Server in order to register a callback which will be executed when the size of the list containing the pending connections grow above a given threshold. This will help triggering alerts, or even restarting the process.