Leave cluster listener up when bad data is sent
Removing the ALPN muxer exposed the underlying listener directly to the
gRPC server, which means that a quirk of the ALPN muxer (not surfacing
errors on accept) turned into the gRPC server receiving errors on
accept, and it turns out if the error does not satisfy a special
temporary interface the listener will be closed.
The obviously better thing to do would be to let the listener close
itself if there was truly a fatal error rather than relying on
undocumented interfaces, but here we are.