[Dubbo-1765] Fix ReferenceBean initialization issue

[ATAKing1023]

What is the purpose of the change

Fixed a bug described in Dubbo-1765

If the provider is temporarily unavailable, the reference bean initialization will failed with exception. However, the initialized flag will be set to true, which will cause spring to hold a NULL_OBJECT cache. The subsequent call will always get a null instance.

This change will allow the consumer to obtain the correct instance when provider become available.

Brief changelog

• Set initialized=false in ReferenceConfig when provider is not available.

Verifying this change

unit test passed

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[codecov-io]

Codecov Report

Merging #1999 into master will increase coverage by 1.22%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master   #1999      +/-   ##
===========================================
+ Coverage     50.98%   52.2%   +1.22%     
- Complexity     4775    4912     +137     
===========================================
  Files           562     562              
  Lines         25218   25249      +31     
  Branches       4449    4450       +1     
===========================================
+ Hits          12857   13181     +324     
+ Misses        10431   10065     -366     
- Partials       1930    2003      +73

Impacted Files	Coverage Δ	Complexity Δ
.../java/org/apache/dubbo/config/ReferenceConfig.java	51.41% <100%> (+3.73%)	43 <0> (+5)	⬆️
...he/dubbo/remoting/transport/netty/NettyClient.java	72.88% <0%> (-8.48%)	12% <0%> (-1%)
...ubbo/rpc/protocol/dubbo/ChannelWrappedInvoker.java	41.66% <0%> (-8.34%)	3% <0%> (ø)
.../dubbo/remoting/transport/netty4/NettyChannel.java	60% <0%> (-6.25%)	21% <0%> (-2%)
...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)
...in/java/org/apache/dubbo/rpc/support/RpcUtils.java	20.77% <0%> (+1.29%)	12% <0%> (+1%)	⬆️
...a/org/apache/dubbo/monitor/dubbo/DubboMonitor.java	89.71% <0%> (+1.86%)	15% <0%> (ø)	⬇️
.../remoting/transport/netty4/NettyServerHandler.java	77.08% <0%> (+4.86%)	8% <0%> (+2%)	⬆️
... and 27 more

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