PG-222: Add benchmark support for measuring hook execution time.

Added a new view 'pg_stat_monitor_hook_stats' that provide execution
time statistics for all hooks installed by the module, following is a
description of the fields:
  -       hook: The hook function name.
  -   min_time: The fastest execution time recorded for the given hook.
  -   max_time: The slowest execution time recorded for the given hook.
  - total_time: Total execution time taken by all calls to the hook.
  -   avg_time: Average execution time of a call to the hook.
  -     ncalls: Total number of calls to the hook.
  - load_comparison: A percentual of time taken by an individual hook
                     compared to every other hook.

To enable benchmark, code must be compiled with -DBENCHMARK flag, this
will make the hook functions to be replaced by a function with the same
name plus a '_benchmark' suffix, e.g. hook_function_benchmark.

The hook_function_benchmark will call the original function and
calculate the amount of time it took to execute, than it will update
statistics for that hook.

hash_query.c

@@ -60,6 +60,10 @@ pgss_startup(void)
 		ResetSharedState(pgss);
 	}
 
+#ifdef BENCHMARK
+	init_hook_stats();
+#endif
+
 	pgss->query_buf_size_bucket = MAX_QUERY_BUF / PGSM_MAX_BUCKETS;
 
 	for (i = 0; i < PGSM_MAX_BUCKETS; i++)