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hs: Don't overwrite DoS parameters on circuit with consensus params

David Goulet requested to merge dgoulet/tor:ticket40109_043_01 into master

Turns out that the HS DoS defenses parameters were overwritten by the consensus parameters everytime a new consensus would arrive.

This means that a service operator can still enable the defenses but as soon as the intro point relay would get a new consensus, they would be overwritten. And at this commit, the network is entirely disabling DoS defenses.

Fix this by introducing an "explicit" flag that indicate if the ESTABLISH_INTRO cell DoS extension set those parameters or not. If set, avoid using the consenus at once.

We are not bumping the protover HSIntro value for this because 0.4.2.x series is EOL in 1 month and thus 0.4.3.x would be the only series with this bug. We are confident that a backport and then upgrade path to the latest 0.4.4.x stable coming up soon is enough to mitigate this problem in the coming months.

It avoids the upgrade path on the service side by keeping the requirement for protover HSIntro=5.

Fixes #40109 (closed)

Signed-off-by: David Goulet

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