Minor changes: fix typos in comments, reset debug level to trace
Fix bugs in creation of identifier graph from JSON delta; add debug
information for delta

* Fix bugs in creation of identifier graph from JSON delta in
ProxyGraphService.insertIdentifierInto(): add new metadata and new links
to identifiers already present in identifier graph

* Add debug information for JSON delta in ProxyGraphService.parseJson()

* Add debug information for graph delta in
Connection.debugGraphContent()

* Minor changes: fix typos in comments