Refresh remote profiles that have been marked as stale, in order to fill the...

Refresh remote profiles that have been marked as stale, in order to fill the user directory. [rei:userdirpriv] (#14756)

* Scaffolding for background process to refresh profiles

* Add scaffolding for background process to refresh profiles for a given server

* Implement the code to select servers to refresh from

* Ensure we don't build up multiple looping calls

* Make `get_profile` able to respect backoffs

* Add logic for refreshing users

* When backing off, schedule a refresh when the backoff is over

* Wake up the background processes when we receive an interesting state event

* Add tests

* Newsfile

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

* Add comment about 1<<62

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Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>