FIXED: MD5 computation of qualified literals may crash.
FIXED: library(semweb/rdf_library) to find input that has a .gz (or other
registered _storage extension_).  Chris Dijkshoorn, Jacco van Ossenbruggen.
ENHANCED: Better message if the Turtle empty prefix is not defined.
FIXED: Make erase_triple() thread-safe.  This is called concurrently when
asserted triples are erased for discarding a transaction.