Parallelizing scheduling of multiple circuits via parallel_map.

[upsideon]

Summary

Fixes #2695 by using parallel_map instead of a list comprehension when scheduling multiple circuits to run on a backend.

Details and comments

A new file was added for unit tests corresponding to qiskit/compiler/scheduler.py, as such a file did not exist previously. The tests confirm that scheduling continues to operate as intended following these changes. Additional tests were added to cover code paths that raise exceptions.

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[mtreinish]

Thanks for taking on this task. Overall the code is straightforward enoguh and LGTM, just a few small inline comments in the test. Also before we merge this can you add a feature release note documenting that schedule() will execute in parallel for >1 circuit now so that we document this change. You can find the details on adding release notes

[coveralls]

Pull Request Test Coverage Report for Build 2461276100

• 19 of 19 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 84.427%

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Totals
Change from base Build 2460921856:	0.01%
Covered Lines:	54686
Relevant Lines:	64773

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💛 - Coveralls

[upsideon]

@mtreinish Would you be willing to give these changes another review? Thank you!

[mtreinish]

Thanks for the updates, this is looking better, just a couple of small inline comments mainly around the new tests and how you're monkey patch sys.

[upsideon]

Thank you for the additional review, @mtreinish! I've addressed your most recent set of comments.

[mtreinish]

LGTM, thanks for making the updates