Imp: some improvements

[fangyincheng]

What this PR does:

About some improvements.

Which issue(s) this PR fixes:

Fixes #79
Fixes #67
Fixes #59
Fixes #57

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

• Improve the way of use in consumer.
• Support none request , none response, nil request, nil response
• Support lowercase function on the provider
• Fix bug about Port Occupancy when multi-service

[gaoxinge]

LGTM

[codecov-io]

Codecov Report

Merging #98 into master will increase coverage by 1.25%.
The diff coverage is 83.69%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ Coverage   62.09%   63.34%   +1.25%     
==========================================
  Files          54       54              
  Lines        3580     3656      +76     
==========================================
+ Hits         2223     2316      +93     
+ Misses       1134     1114      -20     
- Partials      223      226       +3

Impacted Files	Coverage Δ
protocol/dubbo/pool.go	62.01% <ø> (+6.87%)	⬆️
common/proxy/proxy_factory/default.go	100% <ø> (ø)	⬆️
common/logger/logger.go	87.23% <100%> (ø)	⬆️
config/reference_config.go	91.54% <100%> (ø)	⬆️
protocol/dubbo/server.go	71.42% <100%> (-0.41%)	⬇️
config/service_config.go	67.6% <100%> (ø)	⬆️
registry/directory/directory.go	78.3% <100%> (+0.2%)	⬆️
registry/zookeeper/registry.go	63.63% <100%> (ø)	⬆️
common/rpc_service.go	88.54% <100%> (-1.53%)	⬇️
protocol/dubbo/client.go	70.13% <100%> (ø)	⬆️
... and 15 more

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[AlexStocks]

LGTM