Jitter signatures times when adding dynamic records.

Jitter signatures times when adding dynamic records.

When doing regular signing expiry time is jittered to make sure
that the re-signing times are not clumped together. This expands
this behaviour to expiry times of dynamically added records.

When incrementally re-signing a zone use the full jitter range if
the server appears to have been offline for greater than 5 minutes
otherwise use a small jitter range of 3600 seconds.  This will stop
the signatures becoming more clustered if the server has been off
line for a significant period of time (> 5 minutes).

Manually edits: resolve conflicts, replace isc_random_uniform
with isc_random_jitter.

(cherry picked from commit 6b2fd40269a69044ecc13e063fca3eae04be36d1)