Use default asyncio event loop implementation in API consumer threads

[zmedico]

Make the _safe_loop function return an AsyncioEventLoop instance,
so that the default asyncio event loop implementation will be used
in API consumer threads. This is possible because the underlying
asyncio.get_event_loop() function returns a separate event loop for
each thread. The AsyncioEventLoop _run_until_complete method will
now appropriately handle a ValueError from signal.set_wakeup_fd(-1)
if it is not called in the main thread.

For external API consumers calling from a non-main thread, an
asyncio loop must be registered for the current thread, or else an
error will be raised like this:

RuntimeError: There is no current event loop in thread 'Thread-1'.

In order to avoid this RuntimeError, the external API consumer
is responsible for setting an event loop and managing its lifecycle.
For example, this code will set an event loop for the current thread:

asyncio.set_event_loop(asyncio.new_event_loop())

In order to avoid a ResourceWarning, the caller should also close
the corresponding loop before the current thread terminates.

Bug: https://bugs.gentoo.org/758755
Signed-off-by: Zac Medico [email protected]

[zmedico]

If an asyncio loop is not running in the main thread, then API consumer threads will not work in practice with less than python3.8, since https://bugs.python.org/issue35621 makes MultiLoopChildWatcher necessary in order to avoid RuntimeError: Cannot add child handler, the child watcher does not have a loop attached.