site: Randomise key setup retry time

This reduces the chance that retries (at both ends of a link, or
within a single secnet) end up synchronised.  Such synchronisation is
not supposed to matter but in practice there have been some bugs where
it does, and it is undesirable anyway.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
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v2: New patch