Added DBUG_ASSERT_AS_PRINTF compile flag

If compiling a non DBUG binary with
-DDBUG_ASSERT_AS_PRINTF asserts will be
changed to printf + stack trace (of stack
trace are enabled).

- Changed #ifndef DBUG_OFF to
  #ifdef DBUG_ASSERT_EXISTS
  for those DBUG_OFF that was just used to enable
  assert
- Assert checking that could greatly impact
  performance where changed to DBUG_ASSERT_SLOW which
  is not affected by DBUG_ASSERT_AS_PRINTF
- Added one extra option to my_print_stacktrace() to
  get more silent in case of stack trace printing as
  part of assert.

sql/sql_plugin.cc

@@ -1669,7 +1669,7 @@ int plugin_init(int *argc, char **argv, int flags)
     */
     global_system_variables.table_plugin =
       intern_plugin_lock(NULL, plugin_int_to_ref(plugin_ptr));
-      DBUG_ASSERT(plugin_ptr->ref_count == 1);
+    DBUG_SLOW_ASSERT(plugin_ptr->ref_count == 1);
 
   }
   mysql_mutex_unlock(&LOCK_plugin);