Add ability for $out to write to remote hosts

SERVER-18027 Support aggregation $out to sharded collection

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• Closed

SERVER-36123 Reject $out with mode: "replaceCollection" if the output collection is sharded

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• Closed

SERVER-36505 Add testing for $out against a stale mongos

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• Closed

SERVER-36513 Add testing in $out for staleShardVersion errors when writing to remote hosts

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• Closed

SERVER-13201 Allow new Aggregation $merge stage to explicitly name a DB to write to

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• Closed