messages: improve speed of "dynamic fields"
(dynamic fields are things like user/role/channel mentions, which are dynamic on content, or the pinning status of it)
we could insert dynamic fields of a message "hot cache" (but it isn't hot, and it isn't a cache), under a
thought about adding an expiry column to it, but it would just slow things down if someone ever wanted to search things back in time or other usecases.
message edits would involve recalculating all required things, of course. and imo that's fine.