debian visitor: do not unnecessarily process ls-lR files

Currently the Debian visitor seems to download and process the `ls-lR` files. Although these change on an almost daily basis it might be wise to add one extra step to calculate a checksum of the `ls-lR` file that was downloaded and verify if there is a checksum from a previous invocation. If there is none, continue processing. If there is one, and it doesn't match, continue processing. If there is one and it matches, exit.

After processing the checksum should be stored in a persistent (configurable?) location.