Merge branch 'stable-3.1'

* stable-3.1:
  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: I9850c32ed9887a86a0f7ac95a39df8201ff5a8f5