public long getHighestModificationTimestamp() {
        return highestModificationTimestamp;
    }
Returns a unique VDS timestamp so that visiting up to and including that timestamp
will return a state including this operation but not any operations sent to the same distributor
after it. For PUT/UPDATE/REMOVE operations this timestamp will be the timestamp of the operation.

@return Returns the modification timestamp.
documentapi/src/main/java/com/yahoo/documentapi/messagebus/protocol/WriteDocumentReply.java