Title: Support re-routing within the proxy

HTTP filters are allowed to modify HTTP requests in progress. Sometimes, that leads to changing the routing decision (which is known prior to filter execution). Currently, it feels a bit ad-hoc to insert "clear routing table" call since it is not clear how that works with 1) multiple filters trying to clear routing tables 2) per-route config that can change mid-filter chain. I think there are three legitimate cases for this kind of behavior:

1. filters change HTTP requests with no change in routing;
2. filters change HTTP requests, and expect the entire chain of filters to be re-applied including the router (this is a natural expectation since it's logically two proxies, stacked together). Some checks need to be applied to prevent infinite loops. Side effects might happen during filter re-evaluation.
3. filters change HTTP requests, and only the router filter gets re-applied at the end. Filter configuration is not re-evaluated using the new route. We need to clarify what happens to per-route filter configs.

I think 1) and 3) are currently implementable in filters. Is there any support for case 2)? Does option 2) make more sense than 3)?