Sends events to Collector in the background, one at a time, so that a slow or unreachable Collector never slows the app down.

• The queue is bounded (1000 events by default); when full, the oldest event is dropped.
• A 5xx or network error resends the same event (same uuid, so it is counted once) after a backoff of 1s, 2s, 4s, … up to a minute, at most 8 times.
• An event that can't be encoded as JSON is dropped.
• A 429 means "slow down": that event was already counted, so it is dropped, and sending pauses for Retry-After seconds.
• 400, 401, 403, 404, 413 and 422 drop the event; resending can't help.