Merge branch 'stable-3.0' into stable-3.1

* stable-3.0:
  Http: fix generation of the custom readonly message
  Upgrade bazlets to latest stable-2.16 to build with 2.16.18 API
  Bump Bazel version to 3.1.0

Change-Id: Ie36732f7dab5a111b89b661eeed1f8bc94bdc46e