Convert IServerInformation to IFeatureCollection

[Tratcher]

This allows us to standardize feature support across servers. Middleware may also mutate the collection if they augment the server capabilities (e.g. add websockets). This will be used to address #328 by adding a server feature for reading and configuring addresses and/or ports.

Open question: Should server features be defined in their own package or in Http.Features? We could put them in Server.Abstractions.

IApplicationBuilder.Server changes from object to IFeatureCollection
IServerFactory.Initialize returns IFeatureCollection
IServerInformation.Start takes an IFeatureCollection