[BUGFIX] Initialize database connection in ClearCacheService

Without calling initialize() on the DatabaseConnection object
the DBAL the resulting connection configuration will default
to the "native" driver type (MySQL) which results in an error
as there is no MySQL server to contact with the given connection
parameters.

Resolves: #67066
Releases: master, 6.2
Change-Id: Id5d70a122dcca806ee1122cecacad0c8aef27547
Reviewed-on: http://review.typo3.org/39636


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>