Deleting a sub-shard in recovery state can mark parent shard as inactive

When deleting a sub-shard after a failed split operation, the delete shard API unloads the replica cores and then deletes the shard state. But, say there were 2 replicas, if the following sequence occurs:

1. 1st replica got deleted
2. for any reason, the other replica published "state=active"

Then the overseer can switch slice states and put parent shard as inactive and the sub-shards as active.

We should defensively update sub-shard state back to "construction" and only then invoke the unload action on replica cores.