Add new layer to serve highway=motorway_junction nodes (#1119)

This PR adds a layer for `highway=motorway_junction` features.

This implementation of highway exits in the transportation_name layer add to the existing layer table structure, and renames the internal column name from "construction" (which was already overloaded with non-construction usages) to "subclass", which will be less confusing to future developers.  The string 'junction' is used as the universal sub-class for highway exits.

A new documentation PR has been opened at openmaptiles/www.openmaptiles.org#69 to reflect these changes in the documentation.

layers/transportation_name/highway_classification.sql

@@ -0,0 +1,57 @@
+CREATE OR REPLACE FUNCTION highway_to_val(hwy_class varchar)
+RETURNS int
+IMMUTABLE
+LANGUAGE plpgsql
+AS $$
+BEGIN
+  CASE hwy_class
+    WHEN 'motorway'     THEN RETURN 6;
+    WHEN 'trunk'        THEN RETURN 5;
+    WHEN 'primary'      THEN RETURN 4;
+    WHEN 'secondary'    THEN RETURN 3;
+    WHEN 'tertiary'     THEN RETURN 2;
+    WHEN 'unclassified' THEN RETURN 1;
+    else RETURN 0;
+  END CASE;
+END;
+$$;
+
+CREATE OR REPLACE FUNCTION val_to_highway(hwy_val int)
+RETURNS varchar
+IMMUTABLE
+LANGUAGE plpgsql
+AS $$
+BEGIN
+  CASE hwy_val
+    WHEN 6 THEN RETURN 'motorway';
+    WHEN 5 THEN RETURN 'trunk';
+    WHEN 4 THEN RETURN 'primary';
+    WHEN 3 THEN RETURN 'secondary';
+    WHEN 2 THEN RETURN 'tertiary';
+    WHEN 1 THEN RETURN 'unclassified';
+    else RETURN null;
+  END CASE;
+END;
+$$;
+
+CREATE OR REPLACE FUNCTION highest_hwy_sfunc(agg_state varchar, hwy_class varchar)
+RETURNS varchar
+IMMUTABLE
+LANGUAGE plpgsql
+AS $$
+BEGIN
+  RETURN val_to_highway(
+    GREATEST(
+      highway_to_val(agg_state),
+      highway_to_val(hwy_class)
+    )
+  );
+END;
+$$;
+
+DROP AGGREGATE IF EXISTS highest_highway (varchar);
+CREATE AGGREGATE highest_highway (varchar)
+(
+    sfunc = highest_hwy_sfunc,
+    stype = varchar
+);