SERVER-27713 Create OpObservers for config.collections writes to update the in-memory chunk metadata data structures

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• Closed

SERVER-28248 stale secondary pings the primary to refresh chunks and waits for updates to propagate

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• Closed

SERVER-29239 Make shard secondaries refresh from shard persisted metadata store

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• Closed

SERVER-29437 Change ShardCollectionType::refreshSequenceNumber from a long to a ChunkVersion

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• Closed

SERVER-30600 mongos does not detect stale config when clients use non-primary read preferences

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• Closed