Fix a bug with using Eio_File handler in main callback.

When using eio_file_stat_ls() (or any other exposing Eio_File *handler),
it will not be possible to use it from the main callback due to eio
freeing data too fast from the thread.

Moved frees from _eio_file_eina_ls_heavy() to eio_async_end() to be
sure that all the calls to main callback are over.
This implied adding the Eina_Iterator from eina_file_stat_ls to
the _Eio_File_Ls structure.