Internally represent incremental backups as a tree

Right now freezer-agent treats backup increments as a flat list and attempts to resolve each increment's parent based only on filename. When flags like '--always-level' are used, this can create entries on the filesystem (e.g. L0 -> L1 -> L2 -> [L3, L3, L3]) freezer isn't capable of discovering, making it impossible to delete or restore these increments.

Freezer should instead actually keep track of the true parent of each backup (or None, for level 0 backups) and operate on a tree in storage.find_all() so these can be managed properly.