Draft: Milestone Stage

[DyrellC]

Description

A draft of the new pipeline for milestone solidification. This consolidates the services provided in the LatestMilestoneTracker, LatestSolidMilestoneTracker and MilestoneSolidifier into just one class that all utilise the same pool of milestone candidate queues, and submits received milestones immediately on storage to be processed through the MilestonStage. This includes the solidification process introduced in #1646

Fixes #1688

Type of change

• Enhancement (a non-breaking change which adds functionality)

How Has This Been Tested?

• Tested by loading from local snapshots of various depths and ensuring that nodes solidify properly.
• Awaiting buildkite tests for regression.
• Unit tests have passed

Checklist:

• [ ] My code follows the style guidelines for this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes