[server] Process Genesis direct children first in moveThreadsToGenesis
[server] Process Genesis direct children first in moveThreadsToGenesis

Summary: I had the realization that if a child is updated before it has a connection to Genesis, it might be missing `KNOW_OF`, and consequently users may lose memberships.

Test Plan: Will be tested in combination with following diffs

Reviewers: atul

Reviewed By: atul

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1260

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