Respond with 409 to POST '/users/sync' if sync is in progress

When user syncing is in progress, we should respond with an OK. (travis-ci/travis-ci#1569)

This PR implements the latter solution I outlined in https://github.com/travis-ci/travis-ci/issues/1569#issuecomment-33310950. This can alternatively handled in `travis-core`, but I believe this is a better approach.