Fix nondeterminism in multi-Pathfinder

[WardBrian]

Submission Checklist

• Run unit tests: ./runTests.py src/test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary

The current design is such that the order of the single-pathfinder draws is determined by the order their thread completes, so a random seed does not always produce the same output of multi-pathfinder.

More details are given in #3237

Intended Effect

Closes #3237

How to Verify

Side Effects

Documentation

Copyright and Licensing

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)