Support multiple meta servers

[nobodyiam]

Thanks to @linzhouzhi, Apollo now supports configuring multiple meta server addresses, e.g

dev.meta=http://1.2.3.4:8080,http://2.3.4.5:8080

Apollo will select one available meta server randomly to use and will keep checking the servers in background thread to evict if one is unavailable.

However, in production environment, we still suggest using one single domain like http://config.xxx.com (backed by software load balancers like nginx) instead of multiple ip addresses, because the servers might scale or shrink.

[codecov-io]

Codecov Report

Merging #1214 into master will decrease coverage by 12.9%.
The diff coverage is 74.41%.

@@              Coverage Diff              @@
##             master    #1214       +/-   ##
=============================================
- Coverage     48.68%   35.78%   -12.91%     
+ Complexity     1682      128     -1554     
=============================================
  Files           365       40      -325     
  Lines         10181     1062     -9119     
  Branches       1031      134      -897     
=============================================
- Hits           4957      380     -4577     
+ Misses         4859      631     -4228     
+ Partials        365       51      -314

Impacted Files	Coverage Δ	Complexity Δ
.../ctrip/framework/apollo/core/MetaDomainConsts.java	79.16% <74.07%> (-15.58%)	12 <11> (+10)
...com/ctrip/framework/apollo/core/utils/NetUtil.java	75% <75%> (ø)	3 <3> (?)
...framework/apollo/portal/entity/vo/PageSetting.java
...ork/apollo/portal/service/AppNamespaceService.java
...rip/framework/apollo/internals/YamlConfigFile.java
...amework/apollo/portal/service/FavoriteService.java
...llo/portal/spi/defaultimpl/DefaultUserService.java
...mework/apollo/portal/controller/AppController.java
...com/ctrip/framework/apollo/common/dto/BaseDTO.java
...p/framework/apollo/biz/service/ReleaseService.java
... and 320 more

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[coveralls]

Coverage increased (+0.4%) to 52.688% when pulling c242d83b188df4feef38868718a25dd379b193b1 on nobodyiam:multiple-meta-server into 0872057 on ctripcorp:master.