[ENH] Improve backoff and throttling of wal3.

[rescrv]

This does multiple things:

• Adds a Backoff code to pushlogs and plumbs it upwards.
• Coalesces the two timeouts that were used in batch_manager and uses
  them everywhere.
• Stops batching by record count. Byte count only.
• Adds a backoff condition to the batch manager. If the batch size and
  then some is in the log, it'll set backoff mode to shed further
  writes.
• Configures the writer in tilt config.
• Plumbs an error for exp backoff.

[github-actions]

Reviewer Checklist

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Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)