[PATCH v2 4/4] Allow to reuse tuple_formats for ephemeral spaces

[Vladimir Davydov]

Pushed the patch to 2.1 with some minor changes:
 - moved tuple_format_remove_from_hash after tuple_format_add_to_hash;
 - moved tuple_format_cmp and tuple_format_hash to the beginning of
   the file;
 - removed the hunk making tuple_format_field take const format -
   added a cast removing const to tuple_format_cmp instead.