Prime cache with message bodies through background job

Mail doesn't go to IMAP to fetch a list of messages, but we have to contact IMAP for the bodies. This can cause a slight delay when users browser their messages. To improve things we already have mem caching (through Horde) that is supposed to prevent the same message to be fetched twice. However, if the first fetch is triggered by the user, the user will experience the delay.

Therefore I propose that we try to prime the cache with message bodies from the cron background jobs, so that the most recent email (or all new?) are put into the cache.

Of course we might overfetch or underfetch, but that is inevitable. If it helps reduce loading times for 50% of the messages the user will browser then we've already won.

From a technical perspective we have to possibly check what type of cache is used as prefetching and caching on a cache that is not shared between CLI (cron) and web requests won't give any benefit.

No files or tests are named. Start by tracing the cron background jobs and the cache configuration shared between CLI and web requests; done means recent message bodies are primed without relying on a non-shared cache.