Ftr: Nearest first for multiple registry

[cityiron]

What this PR does:
This extension provides a strategy to decide how to distribute traffics among them:

1. registry marked as 'preferred=true' has the highest priority.
2. check the zone the current request belongs, pick the registry that has the same zone first.
3. Evenly balance traffic between all registries based on each registry's weight.
4. Pick anyone that's available.

Which issue(s) this PR fixes:

Fixes #597

Special notes for your reviewer:
@zouyx
Does this PR introduce a user-facing change?:

[zouyx]

Well done bro.

But i think this is a new featrue for dubbo-go , so should change the title base on https://github.com/apache/dubbo-go/blob/master/contributing.md

If not ready for merge , should change the status to draft as below

[cityiron]

https://github.com/apache/dubbo-go/blob/master/contributing.md

OK，I will do better next.

[hxmhlt]

I think it should be a new feature. But the original extension registry_aware_cluster_invoker should be hold back?

[AlexStocks]

Have u finished you pr? Plc merge the latest develop branch to fix the conflict problembs.

[cityiron]

I think it should be a new feature. But the original extension registry_aware_cluster_invoker should be hold back?

This feature base on java implementation，no registryAware but zoneAware only.

[cityiron]

Have u finished you pr? Plc merge the latest develop branch to fix the conflict problembs.

Finish and merge develop branch.

[fangyincheng]

LGTM

[codecov-commenter]

Codecov Report

Merging #659 into develop will increase coverage by 0.34%.
The diff coverage is 74.46%.

@@             Coverage Diff             @@
##           develop     #659      +/-   ##
===========================================
+ Coverage    63.58%   63.93%   +0.34%     
===========================================
  Files          239      238       -1     
  Lines        12650    12487     -163     
===========================================
- Hits          8044     7983      -61     
+ Misses        3821     3740      -81     
+ Partials       785      764      -21     

Impacted Files	Coverage Δ
cluster/loadbalance/util.go	66.66% <61.11%> (-19.05%)	⬇️
cluster/cluster_impl/zone_aware_cluster_invoker.go	68.00% <68.00%> (ø)
cluster/cluster_impl/base_cluster_invoker.go	72.15% <85.71%> (+1.31%)	⬆️
cluster/cluster_impl/failover_cluster.go	100.00% <100.00%> (ø)
cluster/cluster_impl/zone_aware_cluster.go	100.00% <100.00%> (ø)
config/reference_config.go	79.82% <100.00%> (+0.92%)	⬆️
config/registry_config.go	80.64% <100.00%> (+1.33%)	⬆️
cluster/router/tag/router_rule.go	71.42% <0.00%> (-12.79%)	⬇️
config/config_center_config.go	64.28% <0.00%> (-9.05%)	⬇️
config/consumer_config.go	56.25% <0.00%> (ø)
... and 11 more

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[zouyx]

Wait for travis pass

[cityiron]

sample pr: dubbogo/dubbo-samples#6