Small optimizations for the ExtensionManager

[gvkries]

The initialization of the ExtensionManager is synchronous and must always be run before any method can be called. It can therefore be moved into the constructor to avoid a check before any method call.
This is a minor performance optimization, but EnsureInitialized was called at least once during every request and on some requests dozens of times.

I based this optimization on PR #16379.

Note: After discussion, only some minor optimizations are kept here.

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[MikeAlhayek]

Have you done any performance benchmark to understand what is the performance gain here?

[gvkries]

Have you done any performance benchmark to understand what is the performance gain here?

I did not measure this, because I don't think it is worth the effort to test it. Same as e.g. making the classes sealed, which is also hardly measurable inside our code base, but safes some CPU cycles anways. And those tiny optimizations will add up over time.
Additionally, the motivation for this was more to make the code simpler.

[gvkries]

I undid the initialization change and moved it back to the original EnsureInitialize. It's just a lot of work for a c'tor.

So there are only a few optimizations left here, e.g. sealing the class and removing some fields that don't need to be kept alive over the lifetime of the application.