[BUGFIX] Avoid session file race-condition in Install Tool

Implement proper locking when accessing the session data file
to avoid race-conditions, which cause random logouts.

Resolves: #73531
Releases: master, 7.6
Change-Id: Ibc14ff771602601fd38af582c1b8a15e3f6dfb4b
Reviewed-on: https://review.typo3.org/46750


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>