BUG/MINOR: clock: validate that now_offset still applies to the current date

BUG/MINOR: clock: validate that now_offset still applies to the current date

We want to make sure that now_offset is still valid for the current
date: another thread could very well have updated it by detecting a
backwards jump, and at the very same moment the time got fixed again,
that we retrieve and add to the new offset, which results in a larger
jump. Normally, for this to happen, it would mean that before_poll
was also affected by the jump and was detected before and bounded
within 2 seconds, resulting in max 2 seconds perturbations.

Here we try to detect this situation and fall back to re-adjusting the
offset instead.

It's more of a strengthening of what's done by commit e8b1ad4c2b
("BUG/MEDIUM: clock: also update the date offset on time jumps") than a
pure fix, in that the issue was not direclty observed but it's visibly
possible by reading the code, so this should be backported along with
the patch above. This is related to issue GH #2704.

Note that this could be simplified in terms of operations by migrating
the deadlines to nanoseconds, but this was the path to least intrusive
changes.

(cherry picked from commit adaba6f904d11424327f90c3ab4df085c26fc8c4)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>