Updating protobuf

1. Fetch the desired protobuf version and copy it in a folder new_proto under third_party/protobuf.
2. Bazel uses upstream protobuf from source, except for java, as we currently don't build protobuf java when bootstrapping bazel. So instead we use pre-built jars. So build the java proto library from source and in case you cloned an upstream version of protobuf, remove the .git folders:

cd new_proto
bazel build :protobuf_java :protobuf_java_util
cp bazel-bin/libprotobuf_java.jar .
cp bazel-bin/libprotobuf_java_util.jar .
bazel clean --expunge
rm -rf .git .gitignore .gitmodules

3. Modify protobuf's BUILD file to not build java from source, but to use the jars instead. To do that, in the BUILD file delete the rules listed under Java support. Then, from the third_party/protobuf/<old_proto>/BUILD file copy the rules under "Modifications made by bazel" to the new BUILD file. The java rules in there should have the same names as the ones you just deleted under "Java support". You might need to update the names of the jars in the rules sources to the ones you just build.
4. Copy third_party/protobuf/com_google_protobuf_java.BUILD to the new directory.
5. Name the new_proto directory according to the protobuf version number.
6. In third_party/protobuf/BUILD update PROTOBUF_VERSION to the name of the directory you just created.
7. Update this file if you found the :instructions to be wrong or incomplete.

Current protobuf version

The current version of protobuf was obtained from @laszlocsomor's protobuf fork https://github.com/laszlocsomor/protobuf at commit a80186eb10e027. Once https://github.com/google/protobuf/pull/2969 is merged into upstream protobuf, we no longer need to use @laszlocsomor's fork but can directly clone upstream.