OpObserver on config server for inserts to config.shards from old mongos

SERVER-24214 Validate shardIdentity document of proposed shard when adding a shard that already exists

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• Backlog

SERVER-24352 configs initialize sharding awareness on sharding-unaware shards on transition to primary

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• Closed

SERVER-24797 mark shard as state=invalid in config.shards on DuplicateKeyError from shardIdentity upsert

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• Closed