Hoist the loop out of get_request, and make sure that it can be reentered when a request is

half-finished.

Note the the req struct used to be re-initialized AFTER reading pipeline data, so now that we
initialize it from the caller we must be careful not to overwrite the pipeline data.

Also we now need to handle the case where the buffer is already full when get_request is called -
previously this never happened as it was always called with an empty buffer and looped until done.

Now get_request is called in a loop, so the next step is to run the loop on a socket only when poll
signals it is readable.