[TASK] Avoid using $GLOBALS['TYPO3_DB'] in DataHandler

Instead of $GLOBALS['TYPO3_DB'] a type hinted property
$this->databaseConnection is used to gain autocompletion and better IDE
inspection in this class.

Resolves: #63326
Releases: master
Change-Id: I97a7158fd7a19ecb9687e66906392678d53b58f6
Reviewed-on: http://review.typo3.org/34614
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>