fix(aio): keep host_monitoring_v2 monitors per event loop

[AhmadMasry]

Description

Fixes #1287.

With the async API, host_monitoring_v2 monitors were stopped and recreated on every statement when an application runs more than one asyncio event loop (for example one per worker thread):

• every monitored statement runs _cleanup_idle_monitors(), which stops monitors that aren't is_usable();
• AsyncHostMonitorV2.is_usable() returned False whenever the monitor ran on a different loop from the caller's;
• the registry key didn't include the loop, so loops shared and fought over one entry, and the cleanup step stopped other loops' monitors whatever their key.

Each recreation opened a new monitoring connection, and statements still running on the other loop lost failure detection.

Changes in aio/host_monitoring_plugin.py:

• The registry is keyed by (monitor key, event loop). Each loop gets one monitor per host and detection settings and keeps reusing it. One per loop is needed because the monitor is an asyncio task and aborts connections that belong to its loop.
• is_usable() is False only when the monitor is stopped, its task has finished, or its loop is closed. Idle expiry is unchanged, and stopping another loop's idle monitor already goes through cancel_task_threadsafe.
• The module registry is guarded by a lock, as the sync monitor service guards its registry with an RLock.

Compared with sync, which shares one monitor per settings and host across all threads, an application with N event loops now holds N monitoring connections per monitored host, instead of a new one per statement.

Testing:

• New tests in tests/unit/test_aio_host_monitoring_plugin.py:
  • test_event_loops_do_not_stop_each_others_monitors: statements alternating between a loop on another thread and the test's loop keep exactly two monitors, none stopped. Failed before the fix (one monitor left).
  • test_monitor_whose_loop_closed_is_replaced: a monitor whose loop has closed is still stopped and replaced.
• The reproduction in [aio] host_monitoring_v2: event loops stop each other's monitors, recreating them on every statement #1287 (40 statements on 2 loops, 1 host) went from 40 monitors and 40 monitoring connections to 2 and 2, with the in-flight statement's monitor still running. Same with different detection settings per loop.
• Full unit suite: 2,393 passed. flake8, isort and mypy are clean on the changed files.
• Not yet run against a live Aurora cluster.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[AhmadMasry]

Superseded by #1290, which combines this fix with the related async monitoring fixes (#1284, #1276, #1287, #1288) in one PR.