Implemented EZP-18865: split dfs tables

Jira story: https://jira.ez.no/browse/EZP-18865

## Credits

Original PR from @omh : https://github.com/ezsystems/ezpublish-legacy/pull/152.

## Description
Adds configurable variables (`eZDFSClusteringSettings.MetaDataTableNameCache` in `site.ini`, and a `CLUSTER_TABLE_METADATA_CACHE` constant in `config[.cluster].ini`) that, if configured, will make DFS/MySQLi store cache data in a distinct table.

This makes maintenance easier, since the cache table can be truncated in a breeze. It will also improve performances a bit by reducing contention on the tables.

## Testing done

Manual tests. Creating content, clearing view cache, and checking SQL queries.

## TODO

- [x] Add a distinct parameter to enable the feature, using the default table name
- [x] Prepare documentation
- [x] Fix tests...