Merge branch 'stable-3.1' into stable-3.2

* stable-3.1:
  Adapt SNAPSHOT plugin api example to the 3.1 version
  Upgrade bazlets to latest stable-3.1
  Prepare for new gerrit_api snapshot version usage
  Upgrade bazlets to latest stable-3.0
  Upgrade bazlets to latest stable-3.0

Change-Id: If7b6f69d6d8230c12dedfd514f3dbed8ef919876