Event logging

[kazuho]

Each event will contain the following fields:

• timestamp
• level - one of: "packet", "frame", "cc"
  • maybe add "stream" as well?
• event - name of the event "send", "receive", etc., which are level-specific
• rest of the fields are event-specific

We will initially log the events using JSON, but the internal API will be kept neutral to the output format.

[kazuho]

mostly done with #23.

The leftovers are:

• [ ] log address
  • at the moment, the addresses are bound to the connection rather than a "path". We need to change that as well, because probing on a separate "path"