Set javascript callback "terminated" flag to true when recycling

(or terminating) the server.

This will allow background JS threads to terminate when recycling, so the
server doesn't just hang indefinitelyi when recycling.

Add more logging in cleanup() when waiting for children threads to terminate.

Also, eliminate the global 'terminate' variable, answering the question:
  Can this be changed to a if(ws_set!=NULL) check instead?

Yes, yes it can.