Add sync locking for Team Create (#590)

* Add sync locking

* Steal lock from users who left without releasing

* Do not remove lock as unknown instance

* Don't delete non Archivable instance

plugin/src/Reconciler/diff.lua

@@ -115,6 +115,12 @@ local function diff(instanceMap, virtualInstances, rootId)
 			local childId = instanceMap.fromInstances[childInstance]
 
 			if childId == nil then
+				if childInstance.Archivable == false then
+					-- We don't remove instances that aren't going to be saved anyway,
+					-- such as the Rojo session lock value.
+					continue
+				end
+
 				-- This is an existing instance not present in the virtual DOM.
 				-- We can mark it for deletion unless the user has asked us not
 				-- to delete unknown stuff.