additional mypy strictness

additional mypy strictness

enable type checking within untyped defs.  This allowed
some more internals to be fixed up with assertions etc.

some internals that were unnecessary or not even used
at all were removed.  BaseCursorResult was no longer
necessary since we only have one kind of CursorResult
now.  The different ResultProxy subclasses that had
alternate "strategies" dont appear to be used at all
even in 1.4.x, as there's no code that accesses the
_cursor_strategy_cls attribute, which is also removed.
As these were mostly private constructs that weren't
even functioning correctly in any case,
it's fine to remove these over the 2.0 boundary.

Change-Id: Ifd536987d104b1cd8b546cefdbd5c1e5d1801082