The original coding read tuples from workers in round-robin fashion,
but performance testing shows that it works much better to read enough
to empty one queue before moving on to the next.  I believe the
reason for this is that, with the old approach, we could easily wake
up a worker repeatedly to write only one new tuple into the shm_mq
each time.  With this approach, by the time the process gets scheduled,
it has a decent chance of being able to fill the entire buffer in
one go.

Patch by me.  Dilip Kumar helped with performance testing.