Better deal with "lost" connections for async Redis

[kristjanvalur]

ConnectionPool keeps a WeakSet of in_use connections, allowing lost ones to be collected. Collection produces a warning and closes the underlying transport.

Pull Request check-list

Please make sure to review and check all of these items:

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

Description of change

Recent changes, which removed the del handling of lost connections, have illustrated how fragile many applications
are. This PR attempts to improve on this, by better tracking "lost" connections.
See #2995 for various manifestations of this problem.

• ConnectionPool objects now keep a WeakSet of in_use_connections. This allows the garbage collector to find such
  objects if they are not returned to the pool for some reason.
• Connection objects now have a __del__ method which outputs a ResourceWarning in the same way as other IO classes
  in the standard library. For some reason, the underlying "stream" objects do not get garbage collected promptly. The
  Connection will perform a non-async close() call on these connections. This may or may not work, since there may or may
  not be an active event loop in place, but in that case, an error is emitted. In both cases, the developer should become aware
  of there being a problem.
• Redis objects call _close() on their connection objects if they get garbage collected.

NOTE: To see the ResourceWarnings, they need to be enabled. By default they are suppressed. Add, for instance
the -W default::ResourceWarning to the python command line.

[codecov-commenter]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (5391c5f) 91.36% compared to head (c497789) 91.35%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2999      +/-   ##
==========================================
- Coverage   91.36%   91.35%   -0.02%     
==========================================
  Files         126      126              
  Lines       32617    32652      +35     
==========================================
+ Hits        29802    29829      +27     
- Misses       2815     2823       +8     

Files	Coverage Δ
redis/asyncio/client.py	91.99% <100.00%> (-0.15%)	⬇️
redis/asyncio/connection.py	83.90% <90.00%> (+0.24%)	⬆️
tests/test_asyncio/test_connection.py	89.45% <46.15%> (-4.55%)	⬇️

... and 3 files with indirect coverage changes

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