Reuse the old nodes' connections when a cluster topology refresh is being done

[barshaul]

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

Current RedisCluster behavior creates new connections to every node when the cluster topology is refreshed.
A fix was made to reuse the old nodes' connections if they are healthy during a cluster topology refresh to reduce connection storms.

[codecov-commenter]

Codecov Report

Merging #2235 (1bac843) into master (bedf3c8) will increase coverage by 0.00%.
The diff coverage is 93.47%.

@@           Coverage Diff           @@
##           master    #2235   +/-   ##
=======================================
  Coverage   91.83%   91.83%           
=======================================
  Files         108      108           
  Lines       27690    27723   +33     
=======================================
+ Hits        25429    25460   +31     
- Misses       2261     2263    +2     

Impacted Files	Coverage Δ
tests/test_asyncio/test_cluster.py	97.44% <57.14%> (-0.21%)	⬇️
redis/cluster.py	90.30% <100.00%> (+0.30%)	⬆️
tests/test_cluster.py	96.83% <100.00%> (-0.08%)	⬇️
redis/asyncio/connection.py	83.85% <0.00%> (+0.11%)	⬆️

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