BUG/MEDIUM: stream-int: Don't rely on CF_WRITE_PARTIAL to unblock opposite si

BUG/MEDIUM: stream-int: Don't rely on CF_WRITE_PARTIAL to unblock opposite si

In the function stream_int_notify(), when the opposite stream-interface is
blocked because there is no more room into the input buffer, if the flag
CF_WRITE_PARTIAL is set on this buffer, it is unblocked. It is a way to unblock
the reads on the other side because some data was sent.

But it is a problem during the fast-forwarding because only the stream is able
to remove the flag CF_WRITE_PARTIAL. So it is possible to have this flag because
of a previous send while the input buffer of the opposite stream-interface is
now full. In such case, the opposite stream-interface will be woken up for
nothing because its input buffer is full. If the same happens on the opposite
side, we will have a loop consumming all the CPU.

To fix the bug, the opposite side is now only notify if there is some available
room in its input buffer in the function si_cs_send(), so only if some data was
sent.

This patch must be backported to 2.0 and 1.9.