fix(registration): jitter cooldown exit and rate-limit registration retries

[andrewazores]

Based on #858
Depends on #858
See #851

Adds two more behaviours:

1. adds jitter to the cooldown time so that if multiple Agent instances enter failure cooldown around the same time, they don't all exit cooldown at the same moment and flood the Cryostat server. This can happen if the Cryostat server itself has failed, for example.
2. adds a retry rate limit on registration so that the Agent will not re-attempt registration too rapidly, even if it has been pinged by the Cryostat server asking it to refresh its registration.