ObservableDeferred's API is a mess

[richvdh]

... and I hate it.

In particular:

• it doesn't consumeErrors by default, which in 99% of cases leads to log spam about unhandled failures
• it does some awful __getattr__ / __setattr__ hijinks to "transparently" pass through the methods on the underlying Deferred, which makes it hard to type correctly and generally leads to magical behaviour.
• I find it weird that you can fire an ObservableDeferred either by directly calling its callback/errback methods, or by calling them on the deferred you pass into it.

I would like to replace it with a much simpler implementation. Here is a proposal for how we can do it without rewriting everything at once.

• Define an abstract base class which ObservableDeferred implements, called AbstractObservableDeferred, which just exposes the observe method:

  class AbstractObservableDeferred(Generic[T], metaclass=abc.ABCMeta):
      @abc.abstractmethod
      def observe(self) -> "Deferred[T]": ...

• Replace consumers (rather than creators) of ObservableDeferred with AbstractObservableDeferred
• Create a new class SimpleObservableDeferred which implementents AbstractObservableDeferred, and just has a callback, an errback, and an observe method. Both methods should return None.
• Replace uses of ObservableDeferred which currently create a brand new Deferred with SimpleObservableDeferred
• Create a new class DeferredObserver which wraps a SimpleObservableDeferred:

  class DeferredObserver(AbstractObservableDeferred[T]):
      def __init__(self): 
          self._observable = SimpleObservableDeferred[T]()
     def observe(self) -> Deferred[T]:
          return self._observable.observe()
     def chain_to_deferred(self, deferred: Deferred[T]) -> None:
          deferred.addCallbacks(self._observable.callback, self._observable.errback)

• Replace uses of ObservableDeferred which chain to an existing Deferred with DeferredObserver
• Remove now-redundant ObservableDeferred