relay: Send back CONNECTION_REFUSED on reentry

The TORPROTOCOL reason causes the client to close the circuit which is not
what we want because other valid streams might be on it.

Instead, CONNECTION_REFUSED will leave it open but will not allow more streams
to be attached to it. The client then open a new circuit to the destination.

Closes #40270

Signed-off-by: David Goulet <dgoulet@torproject.org>