Scanning a system with many files creates a scan that is unreasonably slow

[Mab879]

Based on a quick strace there is some sort of file system walking.

[vojtapolasek]

AFter investigating, it seems like we should not walk the whole file system by default. We should check for the default location of RPM database. Possibly we should tell the user how to specify a different location if the RPM db is not found.