- More fixes to SQLite "join rewriting"; the fix from :ticket:`2967`

- More fixes to SQLite "join rewriting"; the fix from :ticket:`2967`
implemented right before the release of 0.9.3 affected the case where
a UNION contained nested joins in it.   "Join rewriting" is a feature
with a wide range of possibilities and is the first intricate
"SQL rewriting" feature we've introduced in years, so we're sort of
going through a lot of iterations with it (not unlike eager loading
back in the 0.2/0.3 series, polymorphic loading in 0.4/0.5). We should
be there soon so thanks for bearing with us :).
fixes #2969   re: #2967
- solve the issue of join rewriting inspecting various types of
from objects without using isinstance(), by adding some new
underscored inspection flags to the FromClause hierarchy.