SERVER-18027 Support aggregation $out to sharded collection

• 
• Closed

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

• 
• Closed

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

• 
• Closed

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

• 
• Closed

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

• 
• Closed