Added more descriptive error message of why statement was automaticly dropped

Print information if net_clear() skipped bytes (As this otherwise hides critical timeing bugs)
Added DBUG_ASSERT if we get packets out of order
mysql_change_user() could on error send multiple packets, which caused mysql_client_test to randomly fail

sql/sql_class.cc

@@ -420,6 +420,7 @@ void THD::init_for_queries()
 void THD::change_user(void)
 {
   cleanup();
+  killed= NOT_KILLED;
   cleanup_done= 0;
   init();
   stmt_map.reset();