Repository synchronization between primary and secondary nodes

Implement repository synchronization for Gitlab Geo: #76

• pushes on primary node should notify secondaries
• primary node should buffer notification and send them in batches
• primary node should pool regularly notification queue and notify secondaries in batches
• API endpoint to receive batched notification to update projects
• secondary nodes should not accept push from anyone
• geo nodes should have own ssh keys to be able to sync repositories
• secondary nodes should fetch repositories from primary
• admin screen to manage geo nodes