SC plugin manager and Add service-center error codes.

[little-cui]

New features

1. SC plugin manager.

Changes

1. JAV-486 Changes: Add service-center error codes.

[coveralls]

Coverage increased (+0.2%) to 70.336% when pulling b0dd522 on little-cui:dev into 1626c3d on ServiceComb:master.

[codecov-io]

Codecov Report

Merging #167 into master will increase coverage by 0.29%.
The diff coverage is 59.37%.

@@            Coverage Diff             @@
##           master     #167      +/-   ##
==========================================
+ Coverage   65.76%   66.06%   +0.29%     
==========================================
  Files          18       18              
  Lines        4466     4452      -14     
==========================================
+ Hits         2937     2941       +4     
+ Misses       1343     1329      -14     
+ Partials      186      182       -4

Impacted Files	Coverage Δ
server/service/util/heartbeat_util.go	33.33% <ø> (ø)	⬆️
server/service/util/util.go	100% <ø> (ø)	⬆️
server/service/util/schema_util.go	90% <ø> (ø)	⬆️
server/service/util/rule_util.go	78.32% <ø> (ø)	⬆️
server/service/util/microservice_util.go	61.19% <0%> (ø)	⬆️
server/service/util/domain_util.go	76.71% <100%> (ø)	⬆️
server/service/util/tag_util.go	63.63% <100%> (ø)	⬆️
server/service/governservice.go	69.67% <25%> (+1.08%)	⬆️
server/service/util/dependency.go	47.93% <42.85%> (ø)	⬆️
server/service/util/instance_util.go	50.26% <50%> (ø)	⬆️
... and 7 more

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