implement the replication setup function

Some Nix-based development environment amenities

add an endpoint for enabling replication

Small refactoring and code cleanups

Refactor Service initialization to make it easier to add more services

Generalize the existing hack to integrate our lease maintenance service.
This makes it easily usable for more services.

move testresources to a module where we could share them

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.