fix: health endpoint when using --x-asyncio-reactor

[luislhl]

Motivation

In case the --x-asyncio-reactor option was used to run hathor-core, we would get the following error when sending requests to /v1a/health:

 Traceback (most recent call last):
File "/usr/local/lib/python3.10/site-packages/twisted/web/server.py", line 227, in process
self.render(resrc)
File "/usr/local/lib/python3.10/site-packages/twisted/web/server.py", line 292, in render
body = resrc.render(self)
File "/usr/local/lib/python3.10/site-packages/twisted/web/resource.py", line 268, in render
return m(request)
File "/usr/local/lib/python3.10/site-packages/hathor/healthcheck/resources/healthcheck.py", line 46, in render_GET
status = asyncio.get_event_loop().run_until_complete(healthcheck.run())
File "/usr/local/lib/python3.10/asyncio/base_events.py", line 625, in run_until_complete
self._check_running()
File "/usr/local/lib/python3.10/asyncio/base_events.py", line 584, in _check_running
raise RuntimeError('This event loop is already running')
RuntimeError: This event loop is already running

The reason for this is that basically Twisted would be running its own asyncio event loop already, so we couldn't run another one.

Acceptance Criteria

• We should detect whether there is a running event loop and act accordingly. If there is, we just schedule a task on it. If there isn't, we create a new one to run the healthcheck logic.
• We should now use Deferred's callbacks in this endpoint to handle the response. With them, it becomes easier to adapt to both situations (having a running event loop or not).

Useful References Consulted

• asyncio.ensure_future not working in reactor with inlineCallbacks twisted/twisted#11841
• #11841 fix pending futures twisted/twisted#11890 (comment)

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (0f5ff70) 85.40% compared to head (0f838e0) 85.29%.

Files	Patch %	Lines
hathor/healthcheck/resources/healthcheck.py	88.46%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #940      +/-   ##
==========================================
- Coverage   85.40%   85.29%   -0.11%     
==========================================
  Files         290      290              
  Lines       22460    22477      +17     
  Branches     3379     3380       +1     
==========================================
- Hits        19182    19172      -10     
- Misses       2613     2631      +18     
- Partials      665      674       +9     

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