Keep-Alive and persistent connection in client code

https://github.com/http-rs/async-h1/blob/a1448c379b0cce920e44c62b566bb20db1ce2157/src/client/decode.rs#L28
One point of HTTP/1.1 is persistent connection, but in this line of code, we're stuck with one request/response per connection.

Start at src/client/decode.rs:28 and trace how the client currently handles a request and response before closing the connection. Determine the changes needed for HTTP/1.1 persistent connections; done means multiple request/response exchanges can reuse one connection instead of requiring a new connection for each request.

Written by the indexing model from the issue text.