Feature planning: QLC expansion

[jchristgit]

Our existing QLC implementations allow developers to query and search our cache
for members, guilds, and users.

We could maybe go even further. We could implement QLC tables for querying
state that's retrieved opportunistically from the Discord API or backfilled
into the cache. For instance, a bot that wants to search recent messages for
messages sent by a certain author on a guild could benefit from a message QLC
table which would, in the background, stream the bulk retrieve messages
endpoint.

We should plan out what functionality we envision here and how we want to
expose it to the user.