HTTP2: Enforce independent TCP connections for slow downloads

[share121]

I'm working on a multi-threaded downloader, using the tokio runtime

When the download speed itself is slow

1. When using http2, all threads share a TCP connection, resulting in slow speeds
2. When using http1_only, each thread has an independent TCP connection, which is fast

When the download speed itself is fast

1. When using http2, each thread has an exclusive TCP connection, which is very fast
2. When using http1_only, the speed is much slower than http2

How can I use http2 again so that each thread still has its own TCP connection at low speeds?

[CyanChanges]

ClientBuilder::pool_max_idle_per_host(builder, 0)