Add limit for HTTP/2 settings

We should look at the [IIS per-frame and per-minute HTTP/2 settings limits](https://support.microsoft.com/en-us/help/4491420/define-thresholds-on-the-number-of-http-2-settings-parameters-exchange) and determine how and if to implement something similar in Kestrel.

Kestrel doesn’t limit the number of settings frames or settings parameters per frame. However, with Kestrel’s default max frame size of 16KiB, there cannot be more than ~3000 settings per frame. The cost of processing each setting is also very minimal. Adding a per-frame or even a per-connection limit to Kestrel would be trivial. Adding a per-unit-time limit would be a little trickier.

@muratg @blowdart would like this for 3.0.