Use bigger transactions when doing explicit file system scans

When doing an explicit file system scan we don't need to worry about timeouts so there is no reason to commit the session often to save progress in case the scan aborts.

This provides a large speedup when doing a scan on a lot of small folders such as previews 

cc @PVince81 @DeepDiver1975 
Call it `setUseTransactions($useTransactions)` and add a PHPDoc comment to explain what this is about.