Setup encryption before possible disconnect

[WouterGritter]

There's no reason to delay setting up encryption to after we have fetched the GameProfile from Mojang.

Before this PR, this could result in a disconnect() call before we had set up encryption if Mojang's servers are down:

if (throwable != null) {
  logger.error("Unable to authenticate player", throwable);
  inbound.disconnect(Component.translatable("multiplayer.disconnect.authservers_down"));
  return;
}

// Only here we were setting up encryption

Simulating this with throwable != null || true, we get disconnected on the client with a garbled exception message complaining about packets instead of the proper disconnect message. This is because as far as the client is concerned, encryption should already be in place.

This PR moves enabling encryption on the connection right when we receive the packet and after we're done with setting up the crypto stuff.

This whole code block is already in a giant try/catch block:

try {
  // ...
} catch (GeneralSecurityException e) {
  logger.error("Unable to enable encryption", e);
  mcConnection.close(true);
}

which still results in the same behavior when enableEncryption() fails (error log & disconnect), just without sending an unneeded fetch to Mojang.