[thread-safety] - TaskRegistry, TaskFactoryWrapper

Brief

Fix thread safety issues for the following classes, for the upcoming multi-threaded support.
Check parent issue for more details.
Depends on #11916

Areas

• TaskRegistry.cs

• TaskFactoryWrapper

[ccastanedaucf]

I have an open perf PR that would probably solve this: #12067

IIRC the only true race here was the path that hits the metadata load context since it changes state while ConcurrentDictionary.GetOrAdd doesn't guarantee single execution. The rest was buried in locks, and assembly loads just no-op if run more than once, so a lot of the concurrency stuff was actually not needed (after a fair bit of refactoring).

[surayya-MS]

Couldn't find any thread-safety problem in TaskFactoryWrapper so far. In the prototype it was probably wrongly changed in relation to TaskExecutionHost. Not 100% sure, it will be clear as we start making changes in Tasks as a part of multi-threaded MSBuild effort

[AR-May]

That's possible that we were locking too defensively building the prototype. We can wait for issues to surface when we get multi-threaded tasks.