Waterway zoomlevel 6 - 8 corresponding with water (#1291)

This PR is solving https://github.com/openmaptiles/openmaptiles/issues/406, partially also https://github.com/openmaptiles/openmaptiles/issues/320.
It is changing source of data for zoomlevel 6 - 8 in `waterway` layer to OSM (relation `waterway = river`), this way it corresponds with `water` layer.
Testing on Europe, z0 - z9 tiles increase size from 20.7 MB to 21.4 MB. We can discuss the level of generalization, but I think this size change is not critical.

layers/waterway/mapping.yaml

@@ -42,3 +42,26 @@ tables:
       - canal
       - drain
       - ditch
+
+  # etldoc: imposm3 -> osm_waterway_relation
+  waterway_relation:
+    type: relation_member
+    columns:
+    - name: osm_id
+      type: id
+    - name: member
+      type: member_id
+    - name: role
+      type: member_role 
+    - name: geometry
+      type: geometry
+    - key: waterway
+      name: waterway
+      type: string
+    - key: name
+      name: name
+      type: string
+    - name: tags
+      type: hstore_tags
+    mapping:
+      waterway: [river]