initially working demultiplexer
Treat the disk handler (master) and response handler (slave) distinctly from each other. We can now
flush clients as soon as we're done, without concern from slow IO. Furthermore, we eliminate the gap
between IO and being masterless since we can now block on IO before cleaning up.

In fact, we should consider having master linger around for a while so that any latecomers can
get the file from memory rather than disk.