Consistently use Task or ValueTask in APIs

[lukebakken]

Fixes #1645

• Start with IFrameHandler as pointed out by @danielmarbach

[danielmarbach]

Let's assume the initial design decision was that the ValueTask gets exposed from the bottom layer to the top where the underlying infrastructure like channel is used that already supports ValueTask. If that is true (which I don't know if it is and we might need to ask the original authors of all those changes) then it would make sense to add ValueTask to all the framehandler methods that require it. I haven't looked into details whether it makes sense to do that for close.

Are you following a strategy of doing it "just consistently across the framework handler regardless"?

[lukebakken]

Are you following a strategy of doing it "just consistently across the framework handler regardless"?

Yep! I don't know the rationale behind the API now.

[lukebakken]

@danielmarbach let me know if there are other places in the library you're looking at with regard to this.