In order to be able to remove the DistLock we will have to give more strong guarantees to the allowMigrations flag. The current meaning of this flag when it is unset is:
- ongoing migrations will fail to commit,
- new migrations will not be able to start.
Note that ongoing migrations are not aborted: they will just fail when they tried to commit the moveChunk on the configserver.
The goal of this task is to introduce a way to interrupt ongoing migrations and wait for them.