Fetching role members blocks

Related to #74 #75

When a guild is fetched, the roles are also fetched, which then fetches all the users of the roles. This is obviously not a very good idea.

we need to know the members in each role so we can render the member list.

we could make this process a background task. when a remote guild is joined, fetch the member lists in the background, and if we receive an activity from a unknown member just fetch them now and remove them from the queue.

we should have the unknown member resolution anyway in case of offline guilds?

Start by tracing the guild-fetch path that loads roles and then role users, using related issues #74 and #75 for context. Compare the proposed background fetch and unknown-member resolution behavior; done means guild fetching no longer blocks on all role members while member lists still become available and unknown activity members are resolved.

Written by the indexing model from the issue text.