MEDIUM: lua: do not allocate the remote connection anymore

MEDIUM: lua: do not allocate the remote connection anymore

Lua cosockets do not need to allocate the remote connection anymore.
However this was trickier than expected because some tests were made
on this remote connection's existence to detect establishment instead
of relying on the stream interface's state (which is how it's now done).
The flag SF_ADDR_SET was set a bit too early (before assigning the
address) so this was moved to the right place. It should not have had
any impact beyond confusing debugging.

The only remaining occurrence of the remote connection knowledge now
is for getsockname() which requires to access the connection to send
the syscall, and it's unlikely that we'll need to change this before
QUIC or so.