Add a Tahoe-LAFS replica snapshot downloader

This relates to #235. It integrates the downloader and recoverer together in StatefulRecoverer and implements a Tahoe-LAFS downloader that knows the recovery cap is a directory with a "snapshot.sql" child.

It also adds a bit more to our Tahoe-LAFS HTTP API helper library for creating directories, linking children into them, and retrieving those children again later.

In principle I expect this could actually recover from a snapshot now, except it is not yet plumbed into our HTTP API by _plugin.py (and of course nothing uploads a snapshot yet, and there is no "event stream" handling).