[BUG] don't refresh timeouts late after detected activity

[BUG] don't refresh timeouts late after detected activity

In old versions, before 1.3.16, we had to refresh the timeouts after
each call to process_session() because the stream socket handler did
not do it. Now that the sockets can exchange data for a long period
without calling process_session(), we can detect an old activity and
refresh a timeout long after the last activity, causing too late a
detection of some timeouts.

The fix simply consists in not checking for activity anymore in
stream_sock_data_finish() but only set a timeout if it was not
previously set.