Retry on network errors

[mirukana]

• Client.send() should retry sending requests until it succeeds using an exponential backoff (CTRL+F backoff_factor) algorithm, unless it gets an error that is not possible to simply recover from with time like a 403 Forbidden.
• Matrix rate limit errors should be handled specially, using the retry_after_ms value as the time to wait before retrying.
  Depends on litl/backoff#124