Add parallelism parameter to read_sql() #455

[ipacheco-uy]

What do these changes do?

Add parameter to limit parallel read_sql connections to the minimum of max_sessions, now it is only limited in cpu count.

Related issue number

#455

• passes flake8 modin
• passes black --check modin
• tests added and passing

[codecov]

Codecov Report

Merging #594 into master will decrease coverage by 0.01%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #594      +/-   ##
==========================================
- Coverage   90.42%   90.41%   -0.02%     
==========================================
  Files          37       37              
  Lines        5472     5476       +4     
==========================================
+ Hits         4948     4951       +3     
- Misses        524      525       +1

Impacted Files	Coverage Δ
modin/experimental/pandas/io_exp.py	100% <ø> (ø)	⬆️
modin/experimental/engines/pandas_on_ray/io_exp.py	38.88% <0%> (ø)	⬆️
modin/data_management/factories.py	94.44% <75%> (-0.56%)	⬇️

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[williamma12]

Thanks for the pr, @ipacheco-uy! I think we can ignore the code coverage changes though we might want to use this opportunity to add in a # pragma: no cover to the ExperimentalBaseFactory.

What do you think, @devin-petersohn?

[devin-petersohn]

@williamma12 I think it's a bit out of scope for this PR, we can leave as-is.