MINOR: stconn: Add a field to specify the room needed by the SC to progress

MINOR: stconn: Add a field to specify the room needed by the SC to progress

When the SC is blocked because it is waiting for room in the input buffer,
it will be responsible to specify the minimum free space required to
progress. In this commit, we only introduce the field in the stconn
structure that will be used to store this value. It is a signed value with
the following meaning:

  * -1: The SC is waiting for room but not based on the buffer state. It
        will be typically used during splicing when the pipe is full. In
        this case, only a successful send can unblock the SC.

  * >= 0; The minimum free space in the input buffer to unblock the SC. 0 is
          a special value to specify the SC must be unblocked ASAP, by the
          stream, at the end of process_stream() or when output data are
          consumed on the opposite side.