Mirroring

This might involve:

• accessible update feed
• draft for a mirroring protocol

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Migrated from T1417 (view on Phabricator)

added Sprint 2018 12 label

added priority:Normal label

Feature request from epol on IRC: being able to selectively listen only for the graph and not the object changes. (I guess this will be the case anyway, I'm just mentioning that feedback on the task.)