Add nowait flag to asyncio.Connection.disconnect()

[kristjanvalur]

Pull Request check-list

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• Does $ tox pass with this change (including linting)?
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• Is the new or changed code fully tested?
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Description of change

When the asyncio.Connection object encounters an exception, it often performs a self.disconnect(). This change
adds a nowait flag to this method, so that in the error case, disconnect() does not attempt to await a wait_closed() on the output stream. Awaiting a buffer method at this point can cause other errors to occur.
Notably an asyncio.CancelledError could happen, in case the entire operation is enclosed in a async_timeout.timeout() context manager.

At any rate, if the connection is in an error state, there is no need to wait for the write buffer flush.

This PR is separate from a related pr, #2104, both about making exception handling and cancelling (via Timeout) possible.

[codecov-commenter]

Codecov Report

Base: 92.17% // Head: 92.17% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (ae80bf2) compared to base (9fe8366).
Patch coverage: 89.18% of modified lines in pull request are covered.

❗ Current head ae80bf2 differs from pull request most recent head 6208b66. Consider uploading reports for the commit 6208b66 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2356   +/-   ##
=======================================
  Coverage   92.17%   92.17%           
=======================================
  Files         110      110           
  Lines       28924    28866   -58     
=======================================
- Hits        26661    26608   -53     
+ Misses       2263     2258    -5     

Impacted Files	Coverage Δ
redis/asyncio/connection.py	86.65% <55.55%> (+0.03%)	⬆️
tests/test_asyncio/test_pubsub.py	99.33% <100.00%> (-0.04%)	⬇️
tests/test_pubsub.py	99.59% <0.00%> (-0.02%)	⬇️
tests/test_asyncio/test_cluster.py	97.41% <0.00%> (+0.13%)	⬆️
tests/conftest.py	85.71% <0.00%> (+0.77%)	⬆️
redis/commands/search/aggregation.py	79.72% <0.00%> (+0.84%)	⬆️

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[dvora-h]

@kristjanvalur Can you resolve the conflicts with master?