Do not use file-paths as id values in the database

Derby likes to use dot notation for many things, and file paths have dots—
hence we need to switch to alphanumeric id without dots.

When loading files from disk, we use the same function (from the uuid module)
as is used by derby.js itself, to generate unique id’s.

In all places where the app needs to search by file path, we now rely on the
path attribute instead of the _id attribute