Add config options for recovering corrupted database

[pprishchepa]

These 2 config options are very useful in case we're trying to recover a corrupted database:

• https://mariadb.com/kb/en/library/innodb-system-variables/#innodb_force_recovery
• https://mariadb.com/kb/en/library/innodb-system-variables/#innodb_purge_threads

[csandanov]

was added in https://github.com/wodby/mariadb/releases/tag/3.4.0