[BUGFIX] Better performance of DatabaseConnection->getDateTimeFormats

Currently each call to DatabaseConnection->getDateTimeFormats creates a
new array with default dateTimeFormats. Put this information into a
static class member and return this instead.

Resolves: #63669
Releases: master, 6.2
Change-Id: I457ad6b68fb6919782f349550e9efd07143d0de7
Reviewed-on: http://review.typo3.org/35427
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>