This change splits up the conventional routing path from the attribute
routing path *inside* routing, instead of inside `MvcRouteHandler`. Each
attribute route group now gets its own instance of
`MvcAttributeRouteHandler` which just knows about the actions it can
reach.
This removes the concept of a route-group-token and removes the lookup
table entirely for attribute routing. This also means that the
`DefaultHandler` on `IRouteBuilder` will not be used for attribute routes,
which we are OK with for 1.0.0.
The action selector's functionality is now split into two methods. We
think this is OK for 1.0.0 because any customization of `IActionSelector`
up to now had to implement virtually the same policy as ours in order to
work with attribute routing. It should now be possible to customize the
selector in a meaningful way without interfering with attribute routing.
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