Implemented #18865: Faster cache clearing w/ eZDFS

Separated cache and storage into two DFS tables.
The one for storage isn't changed, while the one for cache
can be customized via constants / INI settings
EZP-18865: CS & optimizations from PR review
EZP-18865: CS, optimizatin and review fixes

- added ezdfsfile_cache table to schema file
- reworked dfsmysqli gateway parameters (no INI loading)
- added missing config.php constants with documentation
- sanitized usage of arguments in mysql.php
- removed include_once in gateway
- removed unused $table properties in gateway
- removed the TABLE_METADATA constant
EZP-18865: Fixed DFS tests
EZP-18865: Enabled feature by default + doc
EZP-18865: review remarks

- clarified some doc + typos
- fixed default cache table in gateway