Consider adding a LazyResettableCts for Kestrel

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 (https://github.com/aspnet/AspNetCore/commit/e04c79bd3dfc0634af1ac6b578124630a5be247d, https://github.com/aspnet/AspNetCore/commit/0622513058eff9a4374a0edca726d2c62959ea1e, 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.