Ease flower server customization

[halsimov]

Describe the type of feature and its functionality.

It would be easier to implement custom Server for example to log some more information if the server class was fully customizable.

Describe step by step what files and adjustments are you planning to include.

I would suggest to move reconnect_client (s), fit_client (s) and evaluate_client (s) functions back to Server class from server.py and have them trasformed as instance methods instead of static functions to be easily overriden if necessary.