XML connector: saves are not atomic, and a failed save is reported as success #160

Found while profiling the XML connector (#157). Line references are to 1abfe74. Reproduced through ConnectorFacade with the test schemas from OpenICF-xml-connector/src/test/resources/test/xml_store.

The connector writes the file in dispose(), which the framework calls after every operation.

• Not atomic. dispose() writes through new FileOutputStream(path) (XMLHandlerImpl.java#L454), which truncates the file before Saxon rewrites it. On a large file this can take seconds (XML connector: every operation reparses and rewrites the whole file, and DOM walks take O(N²) #157). A crash, a kill or a full disk in that window leaves a truncated file. Every later operation then fails with ConnectorException: org.xml.sax.SAXParseException … XML document structures must start and end within the same entity until someone repairs the file.
• Failure reported as success. If the write fails, dispose() throws a ConnectorException (L468-L473), but the framework only logs exceptions from dispose() (ConnectorAPIOperationRunnerProxy.java#L149-L154). The create, update or delete returns success, and its change is lost when the next operation reloads the file. With a read-only file, create returns a uid, and a later search does not find the object.

Writing to a temporary file in the same directory and moving it over the original would avoid the truncation. To report a failed save, the connector has to save before the operation returns, because the framework does not propagate exceptions from dispose().