Use fewer thread locals for MutationGuard

[wolfs]

We now use one ThreadLocal per DefaultMutationGuard instance. This caused a memory leak-like slowdown. We fixed the slowdown by removing the ThreadLocal when possible in #13428.

A better solution would be to have fewer ThreadLocals in the first place, e.g. by only having one ThreadLocal which retains a map of all the mutation guards.

---

cc: @gradle/execution

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Given the limited bandwidth of the team, it will be automatically closed if no further activity occurs. If you're interested in how we try to keep the backlog in a healthy state, please read our blog post on how we refine our backlog. If you feel this is something you could contribute, please have a look at our Contributor Guide. Thank you for your contribution.