Doctor incorrectly reports clock skew

On the consensus-health mails, we're getting lines like this:

NOTICE: The system clock of moria1 is 48 seconds off
NOTICE: The system clock of dizum is 70 seconds off
NOTICE: The system clock of Faravahar is 37 seconds off
NOTICE: The system clock of longclaw is 41 seconds off

But I believe the clocks on these systems are fine.

What I assume is happening is that DocTor is launching a directory fetch for an object like the consensus, and it's taking 48 seconds to finish retrieving the answer, and then DocTor looks at the Date header in the resulting http response, notices that it is from 48 seconds ago, and reports a clock problem.

Here is an improved algorithm: remember when we started the request, and when we finished it, and if the Date header is anywhere within that range, there is nothing to report.

We could instead consider making additional tiny requests where we expect the answer to come back quickly, but I think that might be overkill at this stage.