This object will be needed in a future commit to store requests awaiting
other requests to finish. Doing this in a separate commit just to make
that commit less noisy.
We previously waited until we received all response headers before we
would create the cache entry. We now create one immediately, and handle
writing the headers in its own function. This will allow us to know if
a cache entry writer already exists for a given cache key, and thus
prevent creating a second writer at the same time.
We currently manage request lifetime as both an ActiveRequest structure
and a series of lambda callbacks. In an upcoming patch, we will want to
"pause" a request to de-duplicate equivalent requests, such that only
one request goes over the network and saves its response to the disk
cache.
To make that easier to reason about, this adds a Request class to manage
the lifetime of a request via a state machine. We will now be able to
add a "waiting for disk cache" state to stop the request.
