@Override
    public String toString() {
        return "CachedDataFeedKey{" +
               "dataFeedKey=" + hashedDataFeedKey +
               ", sourceFile=" + sourceFile +
               '}';
    }
@return True if this data feed key has expired
stroom-receive/stroom-receive-common/src/main/java/stroom/receive/common/CachedHashedDataFeedKey.java