If the SQL database classes are not being compiled into VTK directly,
register their callback methods so that they can be registered later.
This is being used in the modularized VTK source tree, the ParseURL
method was made public in the derived classes. Alternatively the classes
could be friends with the callback function, or the superclass could
call ParseURL on the newly constructed database objects.

Change-Id: Iea878da34fb37b8fdf64188e80acca3fee988718