savepoint support was added a year ago as get_last_memroot_block()
and free_all_new_blocks(), but it didn't work and was disabled.

* fix it to work
* instead of freeing the memory, only mark blocks free - this feature
  is supposed to be used inside a loop (otherwise there is no need
  to free anything, end of statement will do it anyway). And freeing
  blocks inside a loop is a bad idea, as they'll be all malloc-ed
  on the next iteration again. So, don't.
* fix a bug in mark_blocks_free() - it doesn't change the number of blocks