http: make RoutingSettings a property of the RequestContext

This is groundwork for upcoming PRs.

/cc @sirthias
> So I propose to change the names like this:
> 
> `withChangedSettings` => `mapSettings`
> `rewriteUnmatchedPath` => `mapUnmatchedPath`

:+1: 

> `withSettings`, `withLog`, `withExecutionContext` => `setSettings`, `setLog`, `setExecutionContext`

Mmh, `withXXX` appears to be the de facto standard for `copy` shortcuts. (See `Uri`, `HttpMessage` and friends.)