Invalidate cached address and exit policy when our address changes

Great point from dmr and others on #25423 that when our address changes we
need to invalidate our cached exit policy. STATUS_SERVER events provide us
with a notification when our address changes.

Besides our exit policy, this also lets us simplify our caching around
addresses. Stem had a special TTL so we wouldn't cache invalid addresses
indefinitely, but with this change we can now replace that with event driven
invalidation (much better!).