Fix issue 2349: Let async HiredisParser finish parsing after a Connection.disconnect()

[kristjanvalur]

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Description of change

This change addresses an issue raised in issue #2349.

It ensures that an async Connection.read_response() call can succeed even if a simultaneous Connection.disconnect()
call is issued. Socket disconnects are graceful in that one closes the send direction and can then wait for any data
in transit to arrive, until an EOF is delivered from the remote side. In asyncio parlance this means that to gracefully close a socket, you close the send stream and can then read from the receive stream until you reach an End Of File.

This change makes sure that the internal BaseParser state is maintained so that it can continue to request and parse chunks for as long as it can. Previously, the HiredisParser would get its internal state cleared immediately, and this would cause the read, in progress, to fail with an unexpected error. Now an ongoing read_response() will continue until it succeeds or encounters an error such as EOF.

A new read_response() issued after a disconnect() call has been issued (but before it completes) will immediately fail, howerver. This is to maintain previous semantics, even though we could simply allow it to go through and fail later when it attemts to read from a closed connection.

[codecov-commenter]

Codecov Report

Base: 92.25% // Head: 92.28% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (4d45a5e) compared to base (9e00b91).
Patch coverage: 100.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2557      +/-   ##
==========================================
+ Coverage   92.25%   92.28%   +0.02%     
==========================================
  Files         115      115              
  Lines       29692    29734      +42     
==========================================
+ Hits        27393    27440      +47     
+ Misses       2299     2294       -5     

Impacted Files	Coverage Δ
redis/asyncio/connection.py	87.62% <100.00%> (+0.12%)	⬆️
tests/test_asyncio/test_connection.py	98.08% <100.00%> (+0.69%)	⬆️
tests/test_cluster.py	96.86% <0.00%> (+0.10%)	⬆️
tests/test_asyncio/test_search.py	98.63% <0.00%> (+0.34%)	⬆️

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[kristjanvalur]

random test failure is random.

[dvora-h]

@kristjanvalur Thanks for this PR! Can you please resolve the conflicts so I can merge it?