* ExtendedCommandGroup forces it to be "backend.ai"

* Currently Python's asyncio does not wait for processing of
  "connection lost" event for all open SSL transports.
  This generates a very long and messy exception traces, which would
  confuse the end-users.
  We apply a hack to mitigate this issue.

* When we call other context managers inside context managers and get
  exceptions during an outer enter handler,
  we should call exit handlers of the inner context managers properly
  once their enter handlers are called.