Pull cache test patterns into reusable contextmanagers

Pull cache testing helper setup/teardown patterns into reusable contextmanagers

Testing Done:
./pants goal test tests/python/pants_test/cache:: passes.

https://travis-ci.org/pantsbuild/pants/builds/39634283 (in progress)

Bugs closed: 757

Reviewed at https://rbcommons.com/s/twitter/r/1267/
Add passthru arg support in the new options system.

Anything after a double-dash is captured as-is. Future changes will
plumb them through to tasks that want them.

This is useful e.g., for passing args through to a tool invoked by a task.

Testing Done:
Ran option tests. CI passed.

Reviewed at https://rbcommons.com/s/twitter/r/1270/
Refactor protobuf parsing logic out of protobuf_gen

This is in preparation of adding support for the Wire generator which also uses .proto files as sources.

Testing Done:
CI is green: https://travis-ci.org/pantsbuild/pants/builds/39766434

Bugs closed: 762

Reviewed at https://rbcommons.com/s/twitter/r/1273/
Correctly parse named_is_latest entries from the pushdb

The named_is_latest entry was being implicitly str()d on the way into the database, but was defaulting to a boolean on the way out. Strings are "truthy", so the "False" string would be treated as true.

Reviewed at https://rbcommons.com/s/twitter/r/1245/
Sync of https://github.com/pantsbuild/pants/commit/a94a392baec3ca0922c0a656362cecb94206466e