Fix for issue 6048, where an apparent race condition occurs when concurrent filesystem caches try to clear expired items. To avoid this causing an exception for a user session, arrange for Filesystem::clearExpired to trigger a 'clearExpired.exception' event. This should give the same functionality as currently exists. Also, have the ExceptionHandler plugin listen for the clearExpired.exception event, and suppress it as per other exceptions.
clean up whitespace which caused php_cs to fail the build. Adjust FileSystem::clearExpired method per feedback from initial pull request
add unit test for clearExpired exception. By setting the cache dir to read-only, unlink() will fail, causing the exception event to be triggered.
Ensure that an empty ArrayObject is passed to triggerException - null does not work
php_cs fixes
set exceptionHandler plugin to not throw exceptions, to prove that the exception event is handled as expected.
Add use statements for plugin and options.
Merge branch 'hotfix/6024'

Close #6024
Merge pull request #6049 from brettmc/6048

Fix for issue 6048
[#6049] CS fixes

- single instead of double quotes
- space following conditional declaration
Merge branch 'hotfix/6049' into develop

Forward port #6049