// The cache is a tree structure where each keeps track of its children.
//
// An uncompacted branch contains a count of the files only directly at the
// branch level, and contains link to children branches or leaves.
//
// The leaves are "compacted" based on a number of properties.
// A compacted leaf contains the totals of all files beneath it.
//
// A leaf is only scanned once every dataUsageUpdateDirCycles,