box/txn: move fiber_set_txn to header

We already have a "get" function in this header which is
named in_txt(). Having both get/set in one place should
be more consistent.

Acked-by: Konstantin Osipov <kostja.osipov@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>