recv_sys_t::mutex and rw_trx_hash_elementi_t::mutex were acquired
in reverse (to recorded) order.

Fixed by releasing recv_sys_t::mutex, before iterating rw_trx_hash.
Statistics gathering doesn't really need recv_sys_t::mutex protection,
since it is always done in one thread (trx_roll_crash_recv_trx) and
thus it can't go wrong.