LastModifiedCheckMiddleware shouldn't hit the database for filesystem scans
LastModifiedCheckMiddleware shouldn't hit the database for filesystem scans

Comparing a file's last modification date to the time the last completed
scan started works just as well as accessing the database -- and, at least
in small-scale tests, appears to be about five times faster! (For now, this
change doesn't affect the web scanner, because the link map is built on
UrlLastModified objects and we don't want to break that functionality.)

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