Datastore: expose AsyncConfigurationCommitCohort into DataBroker

With the new APIs we inadvertently lost the ability for BA applications to prevent prevent writes into the config store.

This will be needed by BUG-1484, where FRM needs to stop users from injecting flow cookies (as those will not be honored anyway). While it does not gate implementation, not having this ability will lead to surprising behavior.

Expose the cohort into binding APIs and the machinery needed to make it work.