grain: replace HasSessionContext interface
grain: replace HasSessionContext interface

...with some wrappers around context.Value.

Apparently, at some point, http.Request grew a context, which offers a
nicer way to handle all this.

Note that none of this has been propagated to websession yet.

zenhack/go.sandstorm,zenhack/go.sandstorm,zenhack/go.sandstorm