Add jitter to IMAP Polling to reduce synchronized load

Description Description I am reporting this from the perspective of an IMAP service provider, rather than as a Paperless-ngx user. We operate a medium-sized email service and have observed recurring peaks in IMAP traffic at exactly ten-minute boundaries (08:00, 08:10, 08:20, etc.). We were able to correlate a significant part of this traffic pattern with customers using Paperless-ngx. The likely cause is the default: This means that independent Paperless-ngx installations using the default configuration perform their IMAP polling at the same wall-clock times. At small scale this is probably insignificant, but at the scale of a shared IMAP provider it creates a thundering-herd effect: many otherwise unrelated users simultaneously generate IMAP connections, searches and other mailbox activity. This produces substantially higher peak load than the same amount of traffic distributed over the ten-minute interval. Important clarification I initially suspected that Paperless-ngx wa…