MAJOR: stream_interface: continue to update data polling flags during handshakes

MAJOR: stream_interface: continue to update data polling flags during handshakes

Since data and socket polling flags were split, it became possible to update
data flags even during handshakes. In fact this is very important otherwise
it is not possible to poll for writes if some data are to be forwarded during
a handshake (eg: data received during an SSL connect).