Gated Deployments: addressing the complexity of schema deployments at scale

Introduces 'Gated Deployments' to address the complexity of multi-dimensional schema changes (multiple changes or multi-sharded databases). Gated Deployments allow for more atomic application of multiple schema changes by running lightweight tasks in parallel after bulk work is done sequentially, and by synchronizing changes across shards before a final, near-simultaneous application. It also introduces user-controlled deployment completion timing, allowing users to stage changes and manually apply them within a controlled window, extending the revert window and providing greater control over schema deployment timing.