…verse proxy

The xhttp client's DialStreamOne, DialStreamUp, and DialPacketUp methods called transport.RoundTrip() synchronously, blocking until the full HTTP response headers were received. This caused a deadlock when behind CDNs
(e.g. Cloudflare, CloudFront) or reverse proxies (e.g. Nginx) that buffer response headers until the origin server sends actual body data:
  1. mihomo client waits for HTTP response headers from CDN
  2. CDN waits for origin server to send response body
  3. Origin server (Xray) waits for client to send VLESS handshake in the request body
  4. Request body can't be written because conn hasn't been returned to the caller yet >> deadlock until timeout

Fix: move RoundTrip into a background goroutine and use httptrace.GotConn to detect TCP connection establishment. Return the conn immediately once TCP is connected, using a new WaitReadCloser type to defer response body reading until the first Read() call. This matches the async pattern used
by Xray-core's splithttp client implementation.

Fixes MetaCubeX#2709

…th async download

When download GET is async, upload POST may arrive at the server before GET creates the session. Using getSession (lookup-only) returns nil and responds 400. Switch to getOrCreateSession to allow either request to create the session on-demand, matching Xray-core's upsertSession behavior.

…meout