Fix long standing issue with resultset growth on repeated execution (GHPR#29)

The "save the SQLA argstack" dirty solution introduced in 975b573a actually
resulted in an infinitely growing nested data structure, containing the
history of *every* reinvocation of the resultset. Scale back the problematic
part, though this is still an interim workaround. A DQ-based stack should
make this much cleaner.
Massive incompatible change of ::BlockRunner internals

It was never documented as usable externally (though folks do use it, sigh)
This last set of changes settles the design for proper documentation and
opening up