Handle vat shutdown while connecting

[talex5]

When a vat is shut down it ends all registered connections and stops accepting new ones. However, if a connection was in progress (e.g. doing the TLS handshake but not yet registered) then we would miss it.

The test_parallel_fails test got upgraded to the stricter await_settled_exn in #233, which was wrong but that wasn't detected due to this bug.