Fix Python bug in async mode

[texodus]

Fixes a bug introduced in #1828. Handler callbacks need to dispatch to the event loop from the framework in order to re-inject responses after PerspectiveManager is invoked, and this loop is captured by the top-level invoking thread via get_event_loop() (in this refactor). However, due to a closure misapplication, this loop was exclusively being captured from the Perspective thread instead.

The impact of this can be seen in the python-tornado-streaming example within a few seconds of a client connecting, or in the trio of tests this PR adds.

ERROR:asyncio:Exception in callback None()
handle: <Handle cancelled>
Traceback (most recent call last):
  File "~/python3.9/asyncio/events.py", line 80, in _run
    self._context.run(self._callback, *self._args)
  File "~/python3.9/site-packages/tornado/platform/asyncio.py", line 189, in _handle_events
    handler_func(fileobj, events)
  File "~/python3.9/site-packages/tornado/iostream.py", line 700, in _handle_events
    self._handle_write()
  File "~/python3.9/site-packages/tornado/iostream.py", line 974, in _handle_write
    self._write_buffer.advance(num_bytes)
  File "~/python3.9/site-packages/tornado/iostream.py", line 183, in advance
    assert 0 < size <= self._size
AssertionError