In the new shard-authoritative world, all chunk operations must be paused while DDL operations are running. This requires a reimplementation of the setAllowMigrations() machinery because, right now, it's only stopping migrations.

A resulting requirement is that chunk operations must not commit while the allowMigrations flag is enabled on the config server. This guarantees that the _shardsvrAllowChunkOperations command can assume no chunk operation will update the collection's ShardVersion after _configsvrSetAllowMigrations has run.