* Add FederationClient interface to federationsender
- Use a shim struct in HTTP mode to keep the same API as `FederationClient`.
- Use `federationsender` instead of `FederationClient` in `keyserver`.
* Pointers not values
* Review comments
* Fix unit tests
* Rejig backoff
* Unbreak test
* Remove debug logs
* Review comments and linting
* Initial persistence of blacklists
* Move statistics folder
* Make MaxFederationRetries configurable
* Set lower failure thresholds for Yggdrasil demos
* Still write events into database for blacklisted hosts (they can be tidied up later)
* Review comments
Neil Alexander
Kegsay