When modifying the list of URLs in a DynamicURLResolver, it's necessary
to walk the chain of parent resolvers and re-populate them. This is
because each spot in the chain maintains caches for lookups further in
the chain.

The way we were doing this was slow. We forced a repopulation of every
level between the root and the DynamicURLResolver. This was slow, and
unnecessary. We're now clearing all private cache state instead, forcing
these to repopulate on demand later and not up-front. That helps greatly
when adding/removing multiple URLs at once, and prevents a lot of
unnecessary premature work.

Djblets, Review Board, and Power Pack unit tests pass.