Identify Moonrise worker threads correctly

[HaHaWTH]

Pull Request

Description

Correctly identify Moonrise worker threads, as it removed previous members exposed in MoonriseCommon, old code nolonger works.

The new approach now gets the PriortisedThreadPool member from MoonriseCommon and calling its getCoreThreads method.

Closes #481.

Checklist

Mandatory checks

• The base branch of this PR is an unreleased version branch (that has a ver/ prefix)
  or is a branch that is intended to be merged into a version branch.
• There are no already existing PRs that provide the same changes.
• The PR is within the scope of Terra (i.e. is something a configurable terrain generator should be doing).
• Changes follow the code style for this project.
• I have read the CONTRIBUTING.md
  document in the root of the git repository.

Types of changes

• Bug Fix
• Build system
• Documentation
• New Feature
• Performance
• Refactoring
• Repository
• Revert
• Style
• Tests
• Translation

Compatibility

• Introduces a breaking change
• Introduces new functionality in a backwards compatible way.
• Introduces bug fixes

Documentation

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

Testing

• I have added tests to cover my changes.
• All new and existing tests passed.

Licensing

• I am the original author of this code, and I am willing to
  release it under GPLv3.
• I am not the original author of this code, but it is in public domain or
  released under GPLv3 or a compatible license.