PrivCount proof of concept: implement debugging for PrivCount

Let's debug PrivCount by splitting statistics into multiple categories:

• public statistics: information that is already public
• private statistics: information that is sensitive

We can safely log public statistics at any log level. We can publish public statistics' unencrypted, per-relay values and noise amounts in extrainfo descriptors.

We need a debug mode to log private statistics above info level. When we are in debug mode, and not using any public authorities, we can publish private statistics' unencrypted, per-relay values and noise amounts in extrainfo descriptors.

We'll work out other debugging mechanisms during the proof of concept process.