Support for caching null values

[54hechuan]

优化缓存括展，支持缓存null值

当前CacheFilter存在无法缓存null值问题

但是某些rpc接口被设计成可能返回空值（比如跟据用户名查询用户信息接口，如果查询不到，返回null）

增加一个内部类包装待缓存的数据

static class ValueWrapper {
	private final Object value;
	public ValueWrapper(Object value){
		this.value = value;
	}
	public Object get() {
		return this.value;
	}
}

Verifying this change

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[carryxyh]

format your code, pls

[carryxyh]

Pls check and fix ci problem.

[diecui1202]

The unit tests failed.

Failed tests: 
  CacheFilterTest.testNull:137 expected:<null> but was:<value1>
  CacheFilterTest.testNull:137 expected:<null> but was:<value1>
  CacheFilterTest.testNull:137 expected:<null> but was:<value1>
Tests in error: 
  CacheFilterTest.testException:124 » HazelcastSerialization Failed to serialize...
  CacheFilterTest.testMethodWithArgs:110 » HazelcastSerialization Failed to seri...
  CacheFilterTest.testNonArgsMethod:97 » HazelcastSerialization Failed to serial...
  CacheFilterTest.testNull:134 » HazelcastSerialization Failed to serialize 'org...

Pls. take a look.

[codecov-io]

Codecov Report

Merging #2480 into master will decrease coverage by 0.11%.
The diff coverage is 77.77%.

@@             Coverage Diff              @@
##             master    #2480      +/-   ##
============================================
- Coverage     55.09%   54.97%   -0.12%     
- Complexity     5270     5279       +9     
============================================
  Files           573      573              
  Lines         25446    25508      +62     
  Branches       4518     4527       +9     
============================================
+ Hits          14019    14023       +4     
- Misses         9335     9389      +54     
- Partials       2092     2096       +4

Impacted Files	Coverage Δ	Complexity Δ
...ava/org/apache/dubbo/cache/filter/CacheFilter.java	76.19% <77.77%> (-3.81%)	5 <0> (-1)
...ache/dubbo/remoting/p2p/support/AbstractGroup.java	45.45% <0%> (-11.37%)	7% <0%> (-1%)
.../dubbo/remoting/transport/netty4/NettyChannel.java	57.64% <0%> (-8.61%)	22% <0%> (-1%)
...ubbo/rpc/protocol/dubbo/ChannelWrappedInvoker.java	41.66% <0%> (-8.34%)	3% <0%> (ø)
...apache/dubbo/rpc/protocol/dubbo/FutureAdapter.java	58.06% <0%> (-6.46%)	3% <0%> (ø)
...src/main/java/org/apache/dubbo/rpc/RpcContext.java	46.62% <0%> (-5.25%)	54% <0%> (ø)
...exchange/support/header/HeaderExchangeChannel.java	79.16% <0%> (-4.35%)	35% <0%> (ø)
...ava/org/apache/dubbo/common/utils/ClassHelper.java	71.83% <0%> (-4.29%)	17% <0%> (ø)
...apache/dubbo/rpc/cluster/support/ClusterUtils.java	63.15% <0%> (-4.19%)	2% <0%> (+1%)
...ing/transport/dispatcher/ChannelEventRunnable.java	79.16% <0%> (-1.69%)	9% <0%> (ø)
... and 31 more

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