Update protobuf version

[zaqqwerty]

Resolves #2238

[maffoo]

We should also update the protobuf version in requirements.txt to match what's in the bazel build. Also note that we have a script dev_tools/build-protos.sh to build protos that uses the grpcio-tools python library, which bundles protoc. I think this is typically what we've been running to regenerate protos and check them in. We have the version of that library pinned in the devtools requirements, though it's not immediately clear what version of protoc that bundles. It looks like we never set up CI jobs to ensure that the checked-in generated code is up to date, but we should set that up (also need to update docs to describe how we're handling protos, because there's potential confusion about whether to use check/bazel or dev_tools/build-protos.sh.

[zaqqwerty]

Thank you for the comments. I had indeed used dev_tools/build-protos.sh to regenerate the protos. I find that the grpcio-tools version in the devtools requirements, 1.21.0, points at protobuf version 3.7.0 (third party tools -> protobuf submodule). I updated requirements accordingly.

I find that the bazel machinery in master is actually not functional, I am working on remedying this.

[Strilanc]

I think we used to check that the bazel build worked, but then it kept breaking for reasons out of our control (e.g. the link where the protobuf installer was located stopped working) so I removed the check. If you can get it working consistently, that would be awesome.

[zaqqwerty]

Made some further changes to address your comments:

• Updated the WORKSPACE and BUILD files with the relevant rules for building version 3.8.0 protos
• Added the script check/build-protos to bazel build both the python and C++ proto rules, and added a stage to the travis build to call the script
• Updated the grpcio-tools version to 1.24.0, which uses proto version 3.8.0; ran /dev_tools/build-protos.sh and checked in the updated protos
• Added a section to docs/development describing how to approach these proto tools

[Strilanc]

This looks good to me. @maffoo you agree? If so automerge it.