MINOR: channel/stconn: Move rto/wto from the channel to the stconn

MINOR: channel/stconn: Move rto/wto from the channel to the stconn

Read and write timeouts concerns the I/O. Thus, it is logical to move it into
the stconn. At the end, the stream is responsible to detect the timeouts. So
it is logcial to have these values in the stconn and not in the SE
descriptor. But it may change depending on the recfactoring.

So, now:
  * scf->rto is used instead of req->rto
  * scf->wto is used instead of res->wto
  * scb->rto is used instead of res->rto
  * scb->wto is used instead of req->wto