Context: #70 and #71

class DeferSend {
  constructor(sendAfterBeingBackgroundedTimeout)
}

Current proposal is to make deferSend a class, and sendAfterBeingBackgroundedTimeout its optional field.

1. Should this be a standalone option in [RequestInit]? But it is not relevant to other existing fetch options.
2. Should the timeout support sending after hidden or pagehide (bfcached)? (Previous discussion in Should pageHideTimeout support additional events #13).
3. Need user input for how desirable for this option.
4. Need better naming suggestion.