Fix canonical_header_id function.

schema.sql

@@ -129,7 +129,7 @@ $$;
 -- Name: canonical_header_id(bigint); Type: FUNCTION; Schema: public; Owner: -
 --
 
-CREATE FUNCTION public.canonical_header_id(height bigint) RETURNS integer
+CREATE FUNCTION public.canonical_header_id(height bigint) RETURNS character varying
     LANGUAGE plpgsql
     AS $$
 DECLARE
@@ -149,13 +149,13 @@ BEGIN
   -- if we have less than 1 header, return NULL
   IF header_count IS NULL OR header_count < 1 THEN
     RETURN NULL;
-  -- if we have one header, return its id
+  -- if we have one header, return its hash
   ELSIF header_count = 1 THEN
-    RETURN headers[1].id;
+    RETURN headers[1].block_hash;
   -- if we have multiple headers we need to determine which one is canonical
   ELSE
     canonical_header = canonical_header_from_array(headers);
-    RETURN canonical_header.id;
+    RETURN canonical_header.block_hash;
   END IF;
 END;
 $$;