logging: add support for hashing data

[dunglas]

Follows #4424, #4425 and #4426. Implements a hash filter and hash actions for the query and cookie filters ask suggested by @wiese in #4424 (comment).

[wiese]

Thank you so much for taking this on, @dunglas!

I can imagine the hash filter become more configurable (hashing algorithm, digest length) in the future, similar to what we see in the replace filter (which has the new value passed to it), for which this is a great basis. This would then also render obsolete any (subjective) concerns about the concrete values used.

[dunglas]

I thought about that but I was wondering if it's really necessary to introduce (and maintain) such features. I fail to see a use case.

[francislavoie]

I agree, I don't think we need to make the hashing configurable. 4 bytes is 8 hex characters, which is plenty. Git gets away with less (usually 7 chars) most of the time. 16 ^ 8 = 4.2 billion. If someone later comes up with a real usecase, we can add it without too much trouble. But I don't think we should implement something early without a good reason.

[mholt]

Thanks Kevin! This looks good to me.