h2: Add a rate limit facility for h/2 RST handling ("Rapid reset" mitigation)

[daghf]

This adds parameters h2_rst_allowance and h2_rst_allowance_period, which govern the rate of which we allow clients to reset h/2 streams.

If the limit is exceeded the connection is closed.

The functionality is currently disabled by default (h2_rst_allowance = 0). This is the kind of mitigation that will always need refinement, so it might be hard to come up with a one-size-fits-all kind of default here. It needs to be generous enough to not impact regular traffic, while at the same time low enough to meaningfully mitigate malicious clients. We may come up with some sort of generous number here, and give a recommendation for users to tune this down if under attack.

Mitigates: #3996

[TomasKorbar]

Hi @daghf, i think default 3600 resets per minute is pretty generous while keeping varnish relatively safe. I tested that this limit keeps memory consumption at acceptable values during rapid reset attack. As you said, if the server is under attack, admins can restrict this to a lower value.

[dridi]

@TomasKorbar thank you for the early testing. Could you please test it in conjunction with #3998 ?

[TomasKorbar]

@dridi sure. Give me some time please.