Minor tweaks from a one-day binge of performance analysis.

The only meaningful change was to insert a sched_yield() in the frisbee
decompressor path.  Apparently, the decompressor can run long enough to
cause the incoming socket buffer to overflow.  I was under the assumption
that the decompressor would not run much longer than a single time slice
(0.001 seconds, about 8 packets) before its priority would force it to
be context switched.  But it was running much longer than that!  Forcing
a periodic yield seems to have taken care of this.

One other cause of retransmitted blocks that I saw was where the server
was taking a long time to read data from a file (up to 0.25 seconds).
This would stall the clients and force them to rerequest blocks (which
they do after about 0.10 seconds).  We can improve on this by splitting
the file reading off to a seperate thread.

Most other changes are related to the event logging code.