Rather than performing this call in the SyncManager, we give each
gossipSyncer the ability to mark the first sync completed. This permits
pinned syncers to contribute towards the rpc-level synced_to_graph
value, allowing the value to be true after the first pinned syncer or
regular syncer complets. Unlinke regular syncers, pinned syncers can
proceed in parallel possibly decreasing the waiting time if consumers
rely on this field before proceeding to load their application.
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