BUG/MEDIUM: connections: Don't forget to detach the connection from the SI.

BUG/MEDIUM: connections: Don't forget to detach the connection from the SI.

When we're deferring the mux choice until the ALPN is negociated, we
attach the connection to the stream_interface until it's done, so that we
can destroy it if something goes wrong and the stream is destroy.
Before calling si_attach_cs() to attach the conn_stream once we have it,
call si_detach_endpoint(), or is_attach_cs() would destroy the connection.