Ftr: Hystrix filter for circuit break and service downgrade

[YGrylls]

What this PR does:

Add an implement of filter for circuit break, based on hystrix-go, see filter/impl/hystrix_filter.go
The config of hystrix is exposed to users in the consumer config. Thus a new field FilterConf is added, just like the protocol conf existed.
The service downgrade can also be customized by users by defining extra filters that handles the error returned from hystrix filter, an example is offered
This filter is designed for both client and server end, now only client works because temporarily filters in server end can not get the return information from callService, see protocol/dubbo/listener OnMessage(). Once they are integrated in the future, the filter can be used in server end just like in client end

Special notes for your reviewer:

Unit test is done, see filter/impl/hystrix_filter_test.go

Does this PR introduce a user-facing change?:

• To use the hystrix filter, add "hystrix_consumer" to the filter of references part in the consumer config
• Config the hystrix in the filter_conf part of the consumer config file as the example does, the config can be set at either service or method level
• An error whitelist can be defined in the config to decide what error will trigger hystrix and what will not
• You can define fallback logic by defining customized filter, see how examples/hystrixfilter/dubbo/with-hystrix-go-client/app/example_fallback_filter.go does. Remember to add your fallback filter JUST BEFORE the hystrix filter

[hxmhlt]

pls resolve the conflicting files

[codecov-io]

Codecov Report

Merging #133 into develop will increase coverage by 1.01%.
The diff coverage is 68.64%.

@@             Coverage Diff             @@
##           develop     #133      +/-   ##
===========================================
+ Coverage    66.85%   67.86%   +1.01%     
===========================================
  Files           93       94       +1     
  Lines         5729     5847     +118     
===========================================
+ Hits          3830     3968     +138     
+ Misses        1491     1466      -25     
- Partials       408      413       +5

Impacted Files	Coverage Δ
config/consumer_config.go	61.7% <ø> (ø)	⬆️
config/provider_config.go	61.76% <ø> (ø)	⬆️
filter/impl/hystrix_filter.go	68.64% <68.64%> (ø)
protocol/dubbo/readwriter.go	74.35% <0%> (+2.56%)	⬆️
protocol/dubbo/codec.go	82.35% <0%> (+8.82%)	⬆️
protocol/dubbo/listener.go	64.81% <0%> (+10.64%)	⬆️
protocol/dubbo/pool.go	75% <0%> (+15.76%)	⬆️

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