MEDIUM: session: prepare to support earlier transitions to the established state

MEDIUM: session: prepare to support earlier transitions to the established state

At the moment it is possible in sess_prepare_conn_req() to switch to the
established state when the target is an applet. But sess_update_stream_int()
will soon also have the ability to set the established state via
connect_server() when a connection is reused, leading to a synchronous
connect.

So prepare the code to handle this SI_ST_ASS -> SI_ST_EST transition, which
really matches what's done in the lower layers.