client side preconnect

We have server side preconnect knobs, for scaling connections based on traffic but not client side knobs where you likely want to just fetch N (6/8) HTTP/1.1 connections if H2 doesn't work. It'd be good to have for H1 only endpoints.

Start by locating the existing server-side preconnect knobs and understanding how they scale connections based on traffic. Define how client-side preconnect should fetch a fixed number of HTTP/1.1 connections, only for endpoints where HTTP/2 does not work, and identify the configuration and tests needed to verify that behavior.

Written by the indexing model from the issue text.