Cleanup: Use both the data document and its MD5 in most places,
         so that much fewer conversions between the two
         have to be performed.
SC: Extracted SPARQL query from LWM_UNPACK.
* FIXED: authority locking for unpacking threads.
* OPTIMIZED: carry a data document's dirty URL -- if any -- around
             i.o. sending additional SPARQL Query requests.
* OPTIMIZED: carry a data document's byte size around i.o. sending
             additional SPARQL Query requests.
DEBUG: Added debug case for HTTP status code of SPARQL Update requests.
* DEBUG: Include MD5 in debug messages.
* FIXED: Non-existing predicate exported.
* FIXED: Declared debug messages as discontiguous.
* FIXED: Posting triples no longer requires the data document
         argument for blank node skolemization (skipped now).
* FIXED: storing that a data document is skipping the cleaning
         phase requires its MD5 as argument.
FIXED: Use datatype-specific lexical-to-value mappings on SPARQL result set literals.
Upped submodule.
FIXED: Resolved conflict with semweb/rdf_db.
Upped submodule.
minor
FIXED: File extensions would not be retrieved using SPARQL queries.
Merge branch 'MD5-URL'