BMM Restart Improvements Part 1. Leader Coordinator Issuing Assignment Tokens

[jzakaryan]

This pull request is part of a series of changes that are meant to improve BMM Restart and make it easier to debug restart failures and trace them to the faulty hosts. It introduces the concept of assignment tokens and adds support for issuing those tokens by the leader coordinator. This feature can be toggled by the newly introduced brooklin.server.coordinator.enableAssignmentTokens configuration.

The subsequent PRs will deal with the following aspects:
Part 2 – Changes to the followers' onAssignmentChange to make them claim the tokens issued by the leader.
Part 3 – Changes to the leader to make it poll the ZooKeeper and wait for the assignment change (stop) to be propagated and executed by the cluster. This will also include code for handling assignment failures and signals to the client that problem(s) occurred in the form of log messages and a new metric.
Part 4 – This one will deal with edge cases and cleanup. More specifically, what happens when a leader fails over during an assignment (assignment token nodes are persistent and their lifecycle is tied to that of the parent datastream).

[ehoner]

I can approve again if you want to change the json serialization.

[ehoner]

Are these setters necessary?

If not.
Can they be removed? (Disregard if they are planned for future use in the future parts.)

If they can be removed.
This object would be immutable, would it be better to remove get from the field accessors? (If the get prefix is a sensitive topic disregard.)

[ehoner]

fwiw. annotations similar to below would be required.

public class Demo {
  private final String name;
  private final Integer age;

  @JsonCreator
  Demo(@JsonProperty("name") String name, @JsonProperty("age") Integer age) {
    this.name = name;
    this.age = age;
  }

  @JsonGetter("name")
  public String name() {
    return name;
  }

  @JsonGetter("age")
  public Integer age() {
    return age;
  }
}

[jzakaryan]

Thanks for sharing the API usage example. With the next set of changes I'll change my JSON class to make use of these and keep the class otherwise immutable.