field.  This is NOT just an aesthetic change, it fixes a once-in-a-blue-
moon race (that happened just yesterday) where the disk write thread
cleared ready and then inprogress as two seperate assignments and got
context switched after the first.  The result was that the net thread
saw a chunk that was inprogress even though all blocks had been received
and it issued a zero-block request.  This triggered an assert in the server.
Note that if we were prissy we would protect chunk buffer accesses with
a mutex, but that is overkill for this single piece of shared state that
can be protected via the "reasonable" atomicity of a write instruction.