Fix EZP-23353: Disconnect cluster DB before forking new process

JIRA: 
https://jira.ez.no/browse/EZP-23353

Problem:
When running `updatesearchindexsolr.php` in a DB-based cluster environment (DFS), 
the cluster DB connection is created before the script forks new process(es), which will cause `MySQL server has gone away` errors.

Fix:
If using a `ezpDatabaseBasedClusterFileHandler`, disconnect before forking.

Tests:
Manual
You can use `eZClusterFileHandler::preFork()` instead of the whole block, that's what it's for :-)