serv.py: Fix hang when spawned dynamically with bitbake

The PRServer has the possibility to hang indefinitely blocking on a
semaphore processing a xmlrpc request to send an event back to the
main bitbake instance.  This was observed during a "bitbake -e" on a
heavily loaded machine and the main bitbake instance and cooker exited
before the PRServer emitted its first log.

The stack trace is provided below as to show what happens every time a
logger.info() is executed in the PRServer.  Not only does it write to
the stream handler but it also tries to send the event to the main
event processor.

    self._notempty.acquire()
    self.queue.put(event)
    _ui_handlers[h].event.send(event)
    fire_ui_handlers(event, d)
    fire(record, None)
    self.emit(record)
    hdlr.handle(record)
    self.callHandlers(record)
    self.handle(record)
    self._log(INFO, msg, args, **kwargs)
    (self.dbfile, self.host, self.port, str(os.getpid())))
    self.work_forever()
    pid = self.daemonize()
    self.prserv.start()
    singleton.start()
    self.prhost = prserv.serv.auto_start(self.data)
    cooker.pre_serve()
    bb.cooker.server_main(self.cooker, self.main)
    self.run()
    code = process_obj._bootstrap()
    self._popen = Popen(self)
    self.serverImpl.start()
    server.detach()
    server = start_server(servermodule, configParams, configuration)
    ret = main()

It was never intended for the PRServer to send its logs anywhere but
its own log file.  The event processing is an artifact of how the
PRServer was forked and it inherits the event log handlers.  The
simple fix is to clean up and purge all the log handlers after the
fork() but before doing any of the typical PRServer work or logging.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>