implement leif's magic-folder data model

[dawuud]

implement leif's magic-folder data model

[leif]

As described ​here.

Note that this is not (yet) the agreed-upon plan for Magic Folder development, but dawuud and I are going to take a shot at it :)

[dawuud]

here's our small progress, modified the Uploader to upload the immutable snapshot object and mutate the user's dmd to link to it. the snapshot object so far is simply an immutable directory which links to an immutable file named "content" and N number of links to parent snapshot objects.

​https://github.com/david415/tahoe-lafs/tree/2712.immutable_snapshot_uploader.0

the next step is to figure out how to persist snapshot objects to disk... though in my humble opinion we should preserve all of the Earth Dragon logic... and therefore we must store the mtime and ctime of the locally written file in addition to snapshot objects.

the snapshot object would be any kind of blob inside the sqlite database... but it's tempting to also just make a Tahoe-LAFS object cache since it could be less code complexity to simply persist the Tahoe-LAFS cryptographic blob to disk.

I think initially we should optimize for the smallest suitable code change to magic_folder.py; This modifying our existing abstract db API in magicfolderdb.py