async_cluster: fix simultaneous initialize

[utkarshgupta137]

Pull Request check-list

• Does $ tox pass with this change (including linting)?
• 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?

Description of change

_initialize should be set to True only after the initialization is complete. Otherwise, it can cause a race condition if the client is executing multiple commands at a time, for eg. in asyncio.gather

[codecov-commenter]

Codecov Report

Merging #2231 (eb65f1a) into master (bedf3c8) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2231      +/-   ##
==========================================
- Coverage   91.83%   91.83%   -0.01%     
==========================================
  Files         108      108              
  Lines       27690    27692       +2     
==========================================
+ Hits        25429    25430       +1     
- Misses       2261     2262       +1     

Impacted Files	Coverage Δ
redis/asyncio/cluster.py	89.91% <100.00%> (ø)
tests/test_asyncio/test_cluster.py	97.64% <100.00%> (+<0.01%)	⬆️
tests/test_asyncio/test_search.py	98.35% <0.00%> (-0.33%)	⬇️
tests/test_cluster.py	96.91% <0.00%> (ø)
redis/asyncio/connection.py	83.85% <0.00%> (+0.11%)	⬆️

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