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/expected/basic.out

@@ -11,19 +11,20 @@ select pg_sleep(.5);
  
 (1 row)
 
-SELECT 1;
- ?column? 
-----------
-        1
+SELECT 1 AS num;
+ num 
+-----
+   1
 (1 row)
 
 SELECT query FROM pg_stat_monitor ORDER BY query COLLATE "C";
-             query              
---------------------------------
- SELECT $1
- SELECT pg_stat_monitor_reset()
+                             query                             
+---------------------------------------------------------------
+ SELECT $1 AS num
+ SELECT pg_stat_monitor_reset();
+ SELECT query FROM pg_stat_monitor ORDER BY query COLLATE "C";
  select pg_sleep($1)
-(3 rows)
+(4 rows)
 
 SELECT pg_stat_monitor_reset();
  pg_stat_monitor_reset