Use a thread-safe Set impl for the TickerTask's tickingLocations

[md5sha256]

Description

Bugfix

Proposed changes

• Replace the use of a HashSet with ConcurrentHashMap#newKeySet in the ticker task to prevent CMEs from ocurring when we are ticking
• Updated relevant javadocs
• Replace getOrDefault creations of HashSet with Collections#emptySet

Related Issues (if applicable)

Fixes #3696

Checklist

• I have fully tested the proposed changes and promise that they will not break everything into chaos.
• I have also tested the proposed changes in combination with various popular addons and can confirm my changes do not break them.
• I have made sure that the proposed changes do not break compatibility across the supported Minecraft versions (1.16.* - 1.20.*).
• I followed the existing code standards and didn't mess up the formatting.
• I did my best to add documentation to any public classes or methods I added.
• I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values
• I added sufficient Unit Tests to cover my code.

[github-actions]

Your Pull Request was automatically labelled as: "✨ Fix"
Thank you for contributing to this project! ❤️

[md5sha256]

Haven't tested this on the server yet but I expect the chance of a major bug ocurring are unlikely.

[github-actions]

Slimefun preview build

A Slimefun preview build is available for testing!
Commit: 29036c19

https://preview-builds.walshy.dev/download/Slimefun/4173/29036c19

Note: This is not a supported build and is only here for the purposes of testing.
Do not run this on a live server and do not report bugs anywhere but this PR!

[WalshyDev]

I looked for the HashSet a while ago and couldn't find it, ty!