[BUGFIX] Reestablish DB connection in long-running tasks

During a long-running task the connection to the database may be
disconnected, if mysql `wait_timeout` is exceeded. This patch makes
sure the connection is reestablished if there has been a connection
before.

Resolves: #69001
Releases: master, 6.2
Change-Id: I7575ae386ec190d60ddceee146133125083dee41
Reviewed-on: http://review.typo3.org/42546


Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Josef Glatz <jousch@gmail.com>
Tested-by: Josef Glatz <jousch@gmail.com>
Reviewed-by: Alexander Schnitzler <typo3@alexanderschnitzler.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>