ha_partition creates temporary ha_XXX objects for its partitions when
performing DDL operations. The objects were created on a MEM_ROOT and
never deleted.
This works as long as ha_XXX objects free all data ha_XXX::close() and
don't rely on a proper destructor invocation. Unfortunately, ha_rocksdb
includes String members which need to be delete'd properly.

Fixed the bug by having ha_partition::~ha_partition delete these temporary
objects.