Merge branch 'stable-2.16' into stable-3.0

* stable-2.16:
  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: I30c7e4454740e0e73c7d759a7a7854bfc0b937ed