sync_stages: fix _VerifyMasterId to respect milestone_version

BUG=chromium:611929
TEST=cidb_integration_test; unit tests

Change-Id: I8a45cd8d907ef51e606d6a2a0f025867573567b5
Reviewed-on: https://chromium-review.googlesource.com/344762
Commit-Ready: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
5 files changed