ClientInterface: add a function to verify (correctly) if user limit was reached ...

ClientInterface: add a function to verify (correctly) if user limit was reached (#6258)

* ClientInterface: add a function to verify (correctly) if user limit was reached

CS_HelloSent is a better indicator of active slots than CS_Created, which are session objects created after init packet reception

Switch existing checks to ClientInterface::isUserLimitReached()

Use range-based for loop for getClientIds() used function too

This will fix #6254 (not the memory overhead if init is flooded)