Fix failures in http-over-capnp-test.
Fix failures in http-over-capnp-test.

`task` needs to be the last member of ServerRequestContextImpl, because when we construct it, we call `service.request()`, which may call back to send() or acceptWebSocket(), which require `replyTask` to be initialized.

mologie/capnproto,mologie/capnproto,mologie/capnproto