Access control for eml2 documents
Access control for eml2 documents is pretty complicated.
In access control submodule, the access rules control the documents access,
this can be handled as same as eml beta6. But in additional meta data part,
user can specify another access control for a subtree of the document. This
control is not base on docid and is base on node id. So we maybe need to
change the table structure of xml_access table: adding a new field named
nodeid which is foreign key of xml_nodes table.
When we access control, it should be base on sql rather than java code.