Notify SkipperModule of state changes of Jzab

[EasonLiao]

In SkipperQueue, we have a pre-applied map object for generating idempotent transactions in preprocess (We apply transaction to it in preprocess callback). But we need to reset the map once Jzab comes back from recovery. We should notify all these modules whenever the state of Jzab changes so
that they can do whatever they want.

Probably we should define the SkipperModule interface to something like :

public abstract class SkipperModule {

  protected final CommandPool commandsPool;

  SkipperModule(CommandPool pool) {
    this.commandsPool = pool;
  }

  abstract ByteBuffer preprocess(ByteBuffer request);

// Callback which will be called once in recovering.
 abstract void recovery();

// Callback which will be called once in broadcasting phase.
 abstract void broadcasting();
}