Fix race in device list refreshing.  Recognise that some wireless networks can get into states where udp comms is blocked to/frm certain devices and change refreshes to retry for a fixed number of iterations.  This should reduce instances where many devices are shown as removed then quickly re-added.  It also increases the delay before clients will be told that any devices have been removed.
Remove logging that turned out too noisy for Trace.  Output another bit of Trace info in error cases too.
Remove unnecessary assertions that various thread pools had less than 10 members.
Merge branch 'master' of ssh://core.linn.co.uk/home/git/ohnet