[keyserver] Move merging of updates to a spec

Summary:
Introduce a const into specs defining which updates should be merged.

Depends on D9659

Test Plan: Generate some updates, reintroduce old code, and compare new with old result using _.isEqual.

Reviewers: kamil, inka, bartek

Reviewed By: kamil

Subscribers: ashoat, wyilio

Differential Revision: https://phab.comm.dev/D9660