(feat) Adds new method stateChanged(peerId, newState) for ReplicatorS…

[killme2008]

Motivation:

Track replicator state #521

Modification:

Adds a new method to ReplicatorStateListener

   /**
         * Called when the replicator state is changed. See {@link ReplicatorState}
         * @param peer the replicator's peer id.
         * @param newState the new replicator state.
         * @since 1.3.6
         */
        default void stateChanged(final PeerId peer, final ReplicatorState newState) {}

And a new enum type ReplicatorState:

     /**
       * Represents state changes in the replicator.
       * @author boyan(boyan@antfin.com)
       *
       */
      enum ReplicatorState{
        /**
         * The replicator is created.
         */
        CREATED,
        /**
         * The replicator is destroyed.
         */
        DESTROYED,
        /**
         * The replicator begins to doing it's job(replicating logs or installing snapshot).
         */
        ONLINE,
        /**
         * The replicaotr is suspended by raft error or lost connection.
         */
        OFFLINE
      }

Users can use this API to track the replicator state for some purposes.

Result:

Fixes #521

[killme2008]

@fengjiachun 这个 check好像不 work 了？

[fengjiachun]

@fengjiachun 这个 check好像不 work 了？

check format 失败，你应该没执行格式化代码

[fengjiachun]

LGTM