Improve federated avatars DNS lookups

[strk]

At the moment enabling FEDERATED_AVATARS results in some delays whenever the server has to provide the URL to the avatar of users whose federated avatar url is not found in the ad-hoc cache (which has 1 day of lifetime).

While this may not be a big deal in a page with a single user, explore/users page can show many users and if these have emails with many different domains the delay sums up until browser possibly times out.

The situation may be improved in various ways:

1. Make sure multiple thread asking for the same domain lookup all queue so that a single lookup is ever performed (but I'd think the operating system should already being taking care of this)
2. Always perform the lookup in the background and save the result in db tables

In any case it would be useful to document the problem with the setting that enables these.

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