The goal is to put the "operationTime" filed in the command response. Therefore it will be

  operationTime: <ClusterTime for this operation> 

The method should return just a clusterTime known to the node, the actual implementation will be done in SERVER-27786.

1. add to commands.[h cpp]

// adds operationTime to command bob
void appendOperationTime(Timestamp operationTime, BSONObjBuilder* resultBob)

2. add to dbcommands.[h cpp]

// computes operationTime 
Timestamp getOperationTime(OperationContext* txn,
                  const rpc::RequestInterface& request)

3. The appendOperationTime should be called in the Command::run() with the inPlaceReplyBob as an argument.

4. Add unit tests