feat enhance : Ignore specified unhealthy tables during load balancing.

[ruojieranyishen]

https://github.com/apache/incubator-pegasus/issues/1570

There are three modifications to implement the issue：

1. Modifiy load balancing start condition.

-    if (healthy_partitions != total_partitions) {
+    if (total_partitions - healthy_partitions != ignore_app_unhealthy_partitions) {

3. Refactor the existing app blacklist, and only transfer the required apps to the load balancer.
4. Add unit tests. app_blacklist_with_balancer_test

[acelyc111]

Hi @ruojieranyishen, thanks for the contribution!

Do you mind to separate the refactor (the point 3 you mentioned above) to another independent pull request? It would be nice the keep the pull requests to be more independent. Thanks!

[ruojieranyishen]

@acelyc111 Thanks, I will take your advice and get it done as soon as possible.