- #590, also related to #561
- move feature interfaces from `Microsoft.AspNetCore.Http` package
- move required classes from `Microsoft.AspNetCore.Http.Abstractions` package
- move `ISession` and `WebSocketAcceptContext` to `Microsoft.AspNetCore.Http` namespace (#590)
nit: remove transient dependencies listed in `Microsoft.AspNetCore.Http.Abstractions`'s `project.json`