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
