Fix server error for events containing values with control characters which did not pass the JSONSchemaValidator

The override_required decorator was implemented instead of using the abc
module due to its instability, however, unlike @abstractmethod from abc
it does allow class instantiation even though the method was not
overriden.

as the json structure stored is not specific to any database management
system.

to prevent clashing of underscores in client names and underscores used by LIKE.

In MYSQL 8.0 one cannot create users using GRANT.
https://dev.mysql.com/doc/refman/8.0/en/mysql-nutshell.html#mysql-nutshell-removals
(MariaDB worked fine)

This did not work for python 3.9 and 3.10 (each for a different reason)
The context management of the cursor, seems to have been an undocumented feature
for at least six years.

Simplify data download and unnecessary config, add CESNET banners and legend, more interesting arcs animation