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.

storage/mroonga/lib/mrn_debug_column_access.cpp

@@ -23,13 +23,13 @@ namespace mrn {
   DebugColumnAccess::DebugColumnAccess(TABLE *table, MY_BITMAP *bitmap)
     : table_(table),
       bitmap_(bitmap) {
-#ifndef DBUG_OFF
+#ifdef DBUG_ASSERT_EXISTS
     map_ = dbug_tmp_use_all_columns(table_, bitmap_);
 #endif
   }
 
   DebugColumnAccess::~DebugColumnAccess() {
-#ifndef DBUG_OFF
+#ifdef DBUG_ASSERT_EXISTS
     dbug_tmp_restore_column_map(bitmap_, map_);
 #endif
   }