A balancer lock in the config.locks collection might be left after a balancer round interrupt (like mongos shutdown). We need a way to tell if it's just an orphaned record and drop if it is.

One possible way to detect orphaned balancer lock is to add a "nonce" field to the document, which will be generated on the mongos startup and persist while mongoS is running - it will tie the lock to the specific incarnation of the specific mongos. Then it's balancer process will be able to detect and remove the orphaned lock.