PG-186: Add support to monitor query execution plan.

This requires refactoring of code to add this functionality. Along with
that this patch contains regression test cases.

regression/sql/top_query.sql

@@ -0,0 +1,19 @@
+CREATE EXTENSION pg_stat_monitor;
+SELECT pg_stat_monitor_reset();
+CREATE OR REPLACE FUNCTION add(int, int) RETURNS INTEGER AS
+$$
+BEGIN
+	return (select $1 + $2);
+END; $$ language plpgsql;
+
+CREATE OR REPLACE function add2(int, int) RETURNS int as
+$$
+BEGIN
+	return add($1,$2);
+END;
+$$ language plpgsql;
+
+SELECT add2(1,2);
+SELECT query, top_query FROM pg_stat_monitor ORDER BY query COLLATE "C";
+SELECT pg_stat_monitor_reset();
+DROP EXTENSION pg_stat_monitor;