Do not allow connections till JoinGame is received.

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ConnectedPlayer.java

@@ -481,7 +481,8 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     private Optional<ConnectionRequestBuilder.Status> checkServer(RegisteredServer server) {
       Preconditions
           .checkState(server instanceof VelocityRegisteredServer, "Not a valid Velocity server.");
-      if (connectionInFlight != null) {
+      if (connectionInFlight != null || (connectedServer != null
+          && !connectedServer.hasCompletedJoin())) {
         return Optional.of(ConnectionRequestBuilder.Status.CONNECTION_IN_PROGRESS);
       }
       if (connectedServer != null && connectedServer.getServer().equals(server)) {