MEDIUM: stream-int: make idle-conns switch to ST_RDY

MEDIUM: stream-int: make idle-conns switch to ST_RDY

The purpose of making idle-conns switch to SI_ST_CON was to make the
transition detectable and the operation retryable in case of connection
error. Now we have the RDY state for this which is much more suitable
since it indicates a validated connection on which we didn't necessarily
send anything yet. This will still lead to a transition to EST while not
requiring unnatural write polling nor connect timeouts.