Implement a 'disabled' mode for collections

Implement a mode, either through going to the Config server, or by putting the DDL participants in a 'disabled' mode so that all CRUD operations for a collection will be rejected, because the collection is undergoing a DDL change.

Given the ability to disable migrations through the allowMigrations:false property (SERVER-50027) it is likely a better option to do the disable directly on the shards, though a mode on the Participant.