add max_clock_skew to HTTPMessageVerifier

[achille-roussel]

This PR adds a max_clock_skew attribute to HTTPMessageVerifier so the verification of timestamps is more tolerant to deltas between clocks of the signer and verifier, which are common when

In #10 (comment), we suggested a 30s default. However, in my experience, this is much higher than common clock deltas, which should usually be less than a few hundred milliseconds in healthy systems. I suggest starting with a 5s default, which should be good enough for most use cases. It will be easy to raise the default if needed in the future; however, lowering it could cause damage to production systems that may have been relying on it.

The tolerance applies to all timestamp verification: creation, expiration, and max age. The original issue I reported only mentioned problems with verifying creation time, but I tried to keep it simple and have all time checks behave the same instead of making special cases.

@kislyuk, let me know if you feel strongly about any of those points, and I'll make the change.

Fixes #10

[kislyuk]

LGTM, don't worry about the test failures, the linters are out of date. I'll fix them and do a release soon.

[achille-roussel]

Thanks for the quick resolution, looking forward to the next release!

[kislyuk]

Released in v0.5.0.