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.

libmysqld/libmysql.c

@@ -4375,7 +4375,7 @@ static void stmt_update_metadata(MYSQL_STMT *stmt, MYSQL_ROWS *data)
   MYSQL_FIELD *field;
   uchar *null_ptr, bit;
   uchar *row= (uchar*) data->data;
-#ifndef DBUG_OFF
+#ifdef DBUG_ASSERT_EXISTS
   uchar *row_end= row + data->length;
 #endif