[r=blake-rouse][bug=][author=allenap] Coalesce multiple concurrent requests for the same boot configuration from the region (pxeconfig).
This should reduce load on the region controller when TFTP clients retry, and make it more likely that we're able to service the client in the end; we'll be serving a boot configuration derived from the first request to the region even if we're replying to a second or subsequent attempt by the client to obtain it, rather than starting from the beginning each time.