[BUGFIX] FileLogWriter ignores log file configuration

If there are several instances of TYPO3\CMS\Core\Log\Writer\FileLogWriter
with different log files configured in $logFile, all log records end up
in one file.

This is caused by improper use of static variable $logFileHandle.
All filehandles except the one of the latest instance are ignored.

Resolves: #48918
Releases: 6.2, 6.1, 6.0
Change-Id: Ie6de5e4789d107b541117daf6c7e9855015e0a46
Reviewed-on: https://review.typo3.org/21258
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Markus Klein
Tested-by: Markus Klein