Read from the same worker repeatedly until it returns no tuple.

Read from the same worker repeatedly until it returns no tuple.

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.