* add expiration jitter
Signed-off-by: Miek Gieben <miek@miek.nl>

* sign: add expiration jitter

This PR adds a expiration jitter to spread out zone re-signing even
more. The max is 5 extra days added when creating the signer for a
specific zone.

Also make the duration* constants private to clean up the godoc for this
plugin.
Signed-off-by: Miek Gieben <miek@miek.nl>