refactor(client): make HttpConnect generic over an HTTP service

Instead of wrapping a connector and doing the HTTP/1 handshake itself,
HttpConnect now wraps any HTTP service, sends the CONNECT request
through it, and returns the upgraded IO. The same type works over
HTTP/1 and HTTP/2, where each tunnel is a single stream.

Driving the connection is now the caller's job, so the inline
connection polling is gone. The returned IO can no longer report the
underlying transport's Connected.

Author: heitzlki