Summary:
When large numbers of documents were restarted simultaneously, they had
a tendency to schedule intervals to begin at roughly the same moment in
time, causing periodic spikes in load. This randomizes the delay of each
interval to help avoid such spikes.
Test Plan: Tested manually.
Reviewers: alexmojaki
Subscribers: alexmojaki
Differential Revision: https://phab.getgrist.com/D3600
