- Fixed a memory leak when `.IOLoop` objects are created and destroyed.
- If `.AsyncTestCase.get_new_ioloop` returns a reference to a
  preexisting event loop (typically when it has been overridden to
  return `.IOLoop.current()`), the test's ``tearDown`` method will not
  close this loop.
- Fixed a confusing error message when the synchronous `.HTTPClient`
  fails to initialize because an event loop is already running.
- `.PeriodicCallback` no longer executes twice in a row due to
  backwards clock adjustments.