Hint files for currently installed packages

This issue should allow detection of currently installed packages simply using hint files and file sizes. It does not go to the full extent of hashing files, but just using filesize as a metric.

If the filesize for a given file that we know should be installed, does not match the current one on the server, we know that we are not in sync with what they have installed, and should flag it as "UPDATE".

Migrated from github's hb-appstore#20