Udapte .proto files to their latest version

Noteworthy changes include:
* From remote_execution.proto:
  - Support for symlinks in build output [1].
  - Operation cancellation clarification [2].
  - Documentation updates.
* From http.proto:
  - Documentation updates.

[1] https://github.com/bazelbuild/remote-apis/pull/28
[2] https://github.com/bazelbuild/remote-apis/pull/30