BUG/MEDIUM: lua: block on remote connection establishment

BUG/MEDIUM: lua: block on remote connection establishment

We used to wait for the other side to be connected, but the blocking
flags were inaccurate. It used to work fine almost by accident before
the stream interface changes. Now we use the new RXBLK_CONN flag to
explicitly subscribe to this event.

Thanks to Adis for reporting the issue, PiBaNL for the test case,
and Olivier for the diagnostic.

No backport is needed.