Magic Folder: improve conflict detection to handle more than 2 clients

[daira]

​https://tahoe-lafs.org/pipermail/tahoe-dev/2015-November/009615.html ​https://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/proposed/magic-folder/multi-party-conflict-detection.rst

[daira]

Suggested design criteria:

1. If before every update of a local file F on client A, A has time to observe and download the most recent update of F (and its ancestor directories), then no updates will be treated as conflicts by any client.

2. Stopping a client and restarting it should not make a difference in terms of eventual consistency.

3. There should be a documented way to purge all versions of a given file. (This need not be straightforward, and will likely require garbage collection.)

[daira]

Assigning to me to write the design tradeoff doc.