fix(sync): sync the reference-version across peers

This allows the merge algorithm to not be lost when receiving changes.
Without this change, the optimistic merge algorithm isn't able to make
the distinction between peers, and features end up duplicated.

umap/static/umap/js/umap.layer.js

@@ -1740,6 +1740,10 @@ U.DataLayer = L.Evented.extend({
         delete data.geojson
       }
       this._reference_version = response.headers.get('X-Datalayer-Version')
+
+      const { engine, subject, metadata } = this.getSyncMetadata()
+      engine.update(subject, metadata, '_reference_version', this._reference_version)
+
       this.setUmapId(data.id)
       this.updateOptions(data)
       this.backupOptions()