public Long getLastModified() {
        return lastModified;
    }
Returns the last modified time of this Document, when stored in persistent storage. This is typically set by the
library that retrieves the Document from persistent storage.

This variable doesn't really belong in document. It is used when retrieving docblocks of documents to be able to
see when documents was last modified in VDS, without having to add modified times separate in the API.

NOTE: This is a transient field, and will not be serialized with a Document (will be null after deserialization).

@return the last modified time of this Document (in milliseconds), or null if unset
document/src/main/java/com/yahoo/document/Document.java