One of the big intended benefits from the Kernel work is the ability to use Http caching logic directly rather than implementing our own caching logic. The Kernel component includes a PHP-space implementation of an Http reverse proxy cache. We want to use that for all kernel-called page segments; that is, the entire page itself plus all of the sub-requests. The easiest way to do that is to move the kernel from the 'kernel' DI entry to some other entry, and then make the 'kernel' entry an instance of the HttpCache object, configured with a Store class of our own that saves stuff to the Drupal cache system.
For now, we should let the HttpCache use whatever logic it uses. We can refine it later after the pieces are in place.
Of course, we'll also want a way to disable that cache object when a site is running behind a real proxy cache like Varnish, which is going to be way faster anyway.
This probably needs to wait for #1595146: Load the HttpKernel from the DI Container