Fix issue 1000: Indexes created on WHERE (apache#1133)

This fixes the issue with applying indexes on the WHERE clause.

It does this by modifying the volatility flag for 3 functions -

    _agtype_build_vertex
    _agtype_build_edge
    _label_name

All three were verified to met the specifications for IMMUTABLE.

age--1.3.0.sql

@@ -367,7 +367,7 @@ AS 'MODULE_PATHNAME';
 CREATE FUNCTION ag_catalog._label_name(graph_oid oid, graphid)
 RETURNS cstring
 LANGUAGE c
-STABLE
+IMMUTABLE
 PARALLEL SAFE
 AS 'MODULE_PATHNAME';
 
@@ -3106,18 +3106,19 @@ AS 'MODULE_PATHNAME';
 CREATE FUNCTION ag_catalog._agtype_build_vertex(graphid, cstring, agtype)
 RETURNS agtype
 LANGUAGE c
-STABLE
+IMMUTABLE
 CALLED ON NULL INPUT
 PARALLEL SAFE
 AS 'MODULE_PATHNAME';
 
 --
 -- agtype - edge
 --
-CREATE FUNCTION ag_catalog._agtype_build_edge(graphid, graphid, graphid, cstring, agtype)
+CREATE FUNCTION ag_catalog._agtype_build_edge(graphid, graphid, graphid,
+                                              cstring, agtype)
 RETURNS agtype
 LANGUAGE c
-STABLE
+IMMUTABLE
 CALLED ON NULL INPUT
 PARALLEL SAFE
 AS 'MODULE_PATHNAME';