Double-print of "New SDK_client connected"

[jleibs]

When using:

  rr.init("rerun_example", spawn=True)
  rr.connect()

The SDK prints out the "New SDK client connected" message TWICE which is unexpected for users:

For exampole:

[2024-09-26T00:08:33Z INFO  re_sdk_comms::server] Hosting a SDK server over TCP at 0.0.0.0:9876. Connect with the Rerun logging SDK.
[2024-09-26T00:08:33Z INFO  re_sdk_comms::server] New SDK client connected from: 127.0.0.1:63884
[2024-09-26T00:08:33Z INFO  re_sdk_comms::server] New SDK client connected from: 127.0.0.1:63885

The rationale comes back to the fact that after spawning we wait for the spawned server to be able to be connected to, which is helpful to prevents other connection-timeout messages down the line. See:

rerun/crates/top/re_sdk/src/spawn.rs

Lines 302 to 314 in bd87d4c

	// Give the newly spawned Rerun Viewer some time to bind.
	//
	// NOTE: The timeout only covers the TCP handshake: if no process is bound to that address
	// at all, the connection will fail immediately, irrelevant of the timeout configuration.
	// For that reason we use an extra loop.
	for i in 0..5 {
	re_log::debug!("connection attempt {}", i + 1);
	if TcpStream::connect_timeout(&connect_addr, Duration::from_secs(1)).is_ok() {
	break;
	}
	std::thread::sleep(Duration::from_millis(100));
	}
	}

This "test connection" causes the spurious connection message from the viewer instance.

Now that we have a more defined connection protocol we should be able to include a payload indicating this is a "test" connection and skip the client connection printout in that case.