idleness is defined as an empty work queue. We still use join/leave
messages, but the join message is so that the client can be informed
of the number of blocks in the file. The leave message is strictly
informational, and includes the elapsed time on the client, so that it
can be written to the log file. If that message is lost, no big deal.
I ran a 6 node test on this new code, and all the clients ran in 174
to 176 seconds, with frisbeed using 1% CPU on average (typically
starts out at about 3%, and quickly drops off to steady state).