Gerrit 3.x rewrite
Since Gerrit 3.x NoteDb replaced the SQL database and all metadata is now stored in Git. The user interface migrated away from GWT to Polymer.
· Upgrade the Docker/Ubuntu image to Ubuntu 18.04
· Upgrade the Docker/CentOS image to CentOS 7.6.1810
注意：v2.16是唯一包含和支持 reviewDB 到 NoteDB 的关键版本
https://gitenterprise.me/2019/05/20/gerrit-v3-0-is-here/
2.16.x may have additional ad-hoc releases for issues related to the migration to NoteDb.
目前待升级的 gerrit 版本为 2.12.7
提前测试
测试原版 gerrit 升级路径，不携带任何数据，为的是快速探索成功路径
升级路径：
2.12.7 -- 2.15.21 -- 2.16.25 -- 3.2.3 -- 3.4.1
参考文档：
https://zhuanlan.zhihu.com/p/371933248
https://www.gerritcodereview.com/releases-readme.html