Qubesd limits max payload to 64kb. Do the same in qubesd-query, to avoid
loading to memory potentially unbounded amount of data that would be
refused later anyway.
Reported by @DemiMarie
Remove intermediate qubesd-query-fast proxy process.
This requires changing socket protocol to match what qrexec is sending
in the header.
FixesQubesOS/qubes-issues#3293