Consider adding a LazyResettableCts for Kestrel

[jkotalik]

We have been bitten in the past in Kestrel by making the RequestAborted token lazy. We have now had 3 attempts to fix our disposal of the RequestAborted token (e04c79b, 0622513, and another one from a while ago that I can't find), and though we think our logic is correct now, it isn't easy to guarantee and verify.

A nice way to simplify this logic is to create a new CTS type that is both lazy and resettable. Internally, it would handle races between calling cancel and dispose and be resettable.