[sync] Add store version to model type store backend

Add the ModelTypeStoreSchemaDescriptor protobuf, and add a basic
framework for performing migration. Add a Paranoid PRESUBMIT test to
make sure the record identifier for the db version won't be reused.

R=pavely@chromium.org

BUG=656737

Review-Url: https://chromiumcodereview.appspot.com/2426613003
Cr-Commit-Position: refs/heads/master@{#426900}