Fixes #478.

[solid210]

1. Delete zk node when invoke ReferenceConfig.destory() which consumer uses ZookeeperRegistry.
2. Remove ConcurrentMap<NotifyListener, ChildListener> in zkListeners if the map is empty(release the memory).

What is the purpose of the change

[Issues-487] Fix 487.

Brief changelog

1. Delete zk node when invoke ReferenceConfig.destory() which consumer uses ZookeeperRegistry.
2. Remove ConcurrentMap<NotifyListener, ChildListener> in zkListeners if the map is empty(release the memory).

Verifying this change

1. How to verify zookeeper node?
Prepare:

• Startup a zookeeper application.
• Create an simple dubbo provider in an application, use zookeeper registry.
• Create an simple dubbo consumer in another application, use zookeeper registry.
• Startup dubbo-admin to monitor provider and consumer node.

Before using this commit:

1. Create a ReferenceConfig with GenericService in consumer application and use the GenericService to invoke any method of the provider.
2. Use any browser to open dubbo-admin page and switch to "consumer" tab, you can see there is a consumer data.
3. Invoke ReferenceConfig#destroy() method, then flush page of dubbo-admin.
4. You can see the consumer data still exist (because zk node still exist).

After using this commit:
Do same operations 1, 2 and 3 above.
4. You can see the consumer data is removed (zk node is removed.).

2. How to verify zkListener?
There is a JUnit Test to verify zkListener

[codecov-io]

Codecov Report

Merging #2150 into master will decrease coverage by 0.08%.
The diff coverage is 36.36%.

@@             Coverage Diff              @@
##             master    #2150      +/-   ##
============================================
- Coverage     54.23%   54.15%   -0.09%     
+ Complexity     5097     5095       -2     
============================================
  Files           559      559              
  Lines         24945    24955      +10     
  Branches       4445     4448       +3     
============================================
- Hits          13530    13515      -15     
- Misses         9377     9398      +21     
- Partials       2038     2042       +4

Impacted Files	Coverage Δ	Complexity Δ
...he/dubbo/registry/zookeeper/ZookeeperRegistry.java	63.69% <36.36%> (-2.3%)	32 <1> (+1)
...he/dubbo/remoting/transport/netty/NettyClient.java	72.88% <0%> (-10.17%)	12% <0%> (-1%)
...org/apache/dubbo/rpc/filter/ActiveLimitFilter.java	83.33% <0%> (-5.56%)	6% <0%> (-1%)
.../dubbo/remoting/transport/netty4/NettyChannel.java	61.25% <0%> (-5%)	22% <0%> (-1%)
...ubbo/rpc/protocol/dubbo/ChannelWrappedInvoker.java	41.66% <0%> (-4.17%)	3% <0%> (ø)
...he/dubbo/remoting/transport/netty/NettyServer.java	67.85% <0%> (-3.58%)	8% <0%> (-1%)
...e/dubbo/remoting/transport/netty4/NettyServer.java	72.13% <0%> (-3.28%)	9% <0%> (-1%)
...bo/remoting/transport/netty/NettyCodecAdapter.java	53.12% <0%> (-1.57%)	3% <0%> (ø)
...dubbo/rpc/protocol/dubbo/CallbackServiceCodec.java	77.2% <0%> (-1.48%)	29% <0%> (ø)
...rpc/protocol/dubbo/telnet/InvokeTelnetHandler.java	55.69% <0%> (+3.79%)	13% <0%> (+2%)	⬆️

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[beiwei30]

#1792 should have fixed this problem, pls. verify with the last code base on branch 2.7