`shards unassign` command no longer stalls

[saterus]

This fixes a bug which caused the shard supervisor to stall indefinitely. If it ran shards unassign and there was nothing to be unassigned, it would never increment the etcd revision with its transaction. Then when we waited to observe our changes as reflected by the etcd mod revision, we would end up waiting until the next arbitrary transaction to complete. Needless to say, this is not the desired behavior.

Also adds a timeout to the command more generally.

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This change is 

[saterus]

@jgraettinger Is this good to go out?