async: Use the `HttpConnect` API for establishing network connections
async: Use the `HttpConnect` API for establishing network connections

The async client is no longer tightly coupled to the clear-text
TcpSream-based IO; it uses the `HttpConnect` API that creates a suitable
`TransportStream`. Therefore, now both the async, as well as the
single-threaded (simple) client can consume HTTPS HTTP/2 connections.

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