Synchronous SiteLoader event handling

Previously a browser crash stalled the entire grab, since events from
pychrome were handled asynchronously in a different thread and
exceptions were not propagated to the main thread.

Now all browser events are stored in a queue and processed by the main
thread, allowing us to handle browser crashes gracefully (more or less).

This made the following additional changes necessary:

- Clear separation between producer (browser) and consumer (WARC, stats,
  …)
- Behavior scripts now yield events as well, instead of accessing the
  WARC writer
- WARC logging was removed (for now) and WARC writer does not require
  serialization any more

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