Merge pull request #309 from PrivateStorageio/235.backup-and-recovery.snapshot-tahoe-lafs-downloader

Add a Tahoe-LAFS replica snapshot downloader

A single step is really not very much stateful coverage.

I supposes

and fix the implementation to read the configuration correctly!  thanks, test.

This test is bogus, though, because the Tahoe-LAFS web API test double
misbehaves for directory entry requests.

The capability encoding misbehavior breaks some of our unit tests which are,
supposedly, using the API correctly.

Add a basic recovery function and integrate it with StatefulRecoverer

Encourage Hypothesis to explore a larger range for NotEnoughTokens

This should reduce the number of spurious changed-coverage reports.

hooray, more than a one-off fix

Use trial multiprocess feature even when collecting coverage

encourage Tahoe-LAFS nodes launched in the test suite to exit eventually

Previously we had it searching [1, inf] to discover the boundary at 2 ** 63.
Now we bias its distribution exponentially and hopefully find the very large
boundary sooner.

Also, remove a data() usage as long as I'm here.

Add automatic retries for some Tahoe-LAFS upload errors

If there are not enough reachable storage servers, try again.  Usually the
client node will manage to connect to some more eventually.

Include linting tools in test requirements.

Add some recovery-related endpoints

this will come in handy for building strategies for testing SQL interactions