mod_muc_mam: Switch to mod_cron for message expiry

mod_mam: Switch to new cron API

core.moduleapi: Add API for adding daily or hourly tasks via mod_cron

mod_cron: Initial commit of periodic task runner A number of modules now have periodic tasks that need to run, e.g. for cleaning out old messages or files. This has highlighted a need for coordinating and optimizing scheduling of such tasks.

mod_smacks: Remove obsolete comment It dates back to the initial commit in prosody-modules 9a7671720dec

mod_smacks: Avoid duplicated queueing using flag on session instead of stanza Mutating the stanza like this is Really Bad Practice.

mod_smacks: Remove dead conditional `cached_stanza` can't be falsy because util.stanza.clone() would have thrown rather than return nil, or `._cached=true` would have.

mod_smacks: Ensure hibernating session is not connected Turns out that if you destroy a session from inside prosody, it goes into hibernation but stays connected and continues processing stanzas.

mod_smacks: Reorder imports etc Mostly sorted by name of import (sort -k4) and grouped by kind

mod_smacks: Fix empty cache check In Lua, every value except `nil` and `false` are considered true in boolean expressions, even `0` as here, which means this was dead code.