Allow custom session key generators

[ghost]

Currently, there's no way to implement your own session key generator which is rather restrictive. It'd be nice to have the possibility to provide your own generator or use the built-in generator by default.

[romshark]

As of commit #a1ec834 support for custom session key generators is fully implemented and tested.

A custom key generator can now be passed to the new server hook OnSessionKeyGeneration.

Even though it's possible now to define custom session key generators you shouldn't use this hook, unless you really know what you do as this could compromise security if implemented improperly! The default built-in solution should usually fit any project.