[MEDIUM] split stream_sock_process_data

[MEDIUM] split stream_sock_process_data

It was a waste to constantly update the file descriptor's status
and timeouts during a flags update. So stream_sock_process_data
has been slit in two parts :
  stream_sock_data_update()  => computes updated flags
  stream_sock_data_finish()  => computes timeouts

Only the first one is called during flag updates. The second one
is only called upon completion. The number of calls to fd_set/fd_clr
has now significantly dropped.

Also, it's useless to check for errors and timeouts in the
process_session() loop, it's enough to check for them at the
beginning.