Rework webserver implementation in Auth

This is the first part of a larger rework, which will hopefully end in having a "nice" JSON/REST API in PowerDNS.
Recursor stays untouched for now, while I figure out how to work around locking issues and so on.

Contents of this PR:
* Switch to YaHTTP
* Rework Session/Server classes
* First bunch of new URLs (responses are still in old format)
* Cleanups