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.

mod_smacks: Use stanza type checking function for correctness Non-stanza tables with an 'attr' field might be rare in Prosody, but better to be Correct

mod_smacks: Remove redundant fields Given that the registry is scoped per user and the module is scoped per host, there seems no point to checking or storing both username and host here.