Fixed MovedError, and stopped iterating through startup nodes when slots are fully covered

[barshaul]

…ion of the nodes manager to stop iterating through startup nodes when the coverage is full

Pull Request check-list

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

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Description of change

1. Fixed 'MovedError' bug in RedisCluster: when we reinitialized the nodes topology and then continue to try to execute the command with the new caches we haven't refreshed the target_node and therefore we faced another MovedError.
2. Changed the NodesManager initialize() function to stop iterating through startup nodes when the slots are fully covered

[codecov-commenter]

Codecov Report

Merging #1819 (21ff2b4) into master (4831034) will decrease coverage by 0.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #1819      +/-   ##
==========================================
- Coverage   94.32%   94.30%   -0.02%     
==========================================
  Files          75       75              
  Lines       15783    15788       +5     
==========================================
+ Hits        14887    14889       +2     
- Misses        896      899       +3     

Impacted Files	Coverage Δ
redis/cluster.py	90.23% <85.71%> (-0.07%)	⬇️
tests/test_cluster.py	98.67% <0.00%> (-0.13%)	⬇️

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