server implementations should be able to specify a root context other than `context.Background()`

[hdonnay]

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While looking at adding a capnproto-rpc interface to a project, I came to the conclusion that there's no equivalent to http.Server's BaseContext/ConnContext. This makes context propagation a chore, as a parent context would need to be passed in whatever concrete type is being used and then every method would need to use some tricky code to tie the signalling between the parent and request contexts together.