WindowManager setting of timestamp can encounter race condition with concurrent workers.

```
2013-02-23 14:42:08+0000 [VumiRedis,client] Unhandled error in Deferred:
2013-02-23 14:42:08+0000 [VumiRedis,client] Unhandled Error
        Traceback (most recent call last):
          File "/var/praekelt/vumi-go/ve/lib/python2.6/site-packages/twisted/internet/defer.py", line 397, in errback
            self._startRunCallbacks(fail)
          File "/var/praekelt/vumi-go/ve/lib/python2.6/site-packages/twisted/internet/defer.py", line 464, in _startRunCallbacks
            self._runCallbacks()
          File "/var/praekelt/vumi-go/ve/lib/python2.6/site-packages/twisted/internet/defer.py", line 551, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/var/praekelt/vumi-go/ve/lib/python2.6/site-packages/twisted/internet/defer.py", line 1101, in gotResult
            _inlineCallbacks(r, g, deferred)
        --- <exception caught here> ---
          File "/var/praekelt/vumi-go/ve/lib/python2.6/site-packages/twisted/internet/defer.py", line 1043, in _inlineCallbacks
            result = result.throwExceptionIntoGenerator(g)
          File "/var/praekelt/vumi-go/ve/lib/python2.6/site-packages/twisted/python/failure.py", line 382, in throwExceptionIntoGenerator
            return g.throw(self.type, self.value, self.tb)
          File "/var/praekelt/vumi-go/ve/src/vumi/vumi/components/window_manager.py", line 190, in _monitor_windows
            key = (yield self.get_next_key(window_id))
          File "/var/praekelt/vumi-go/ve/lib/python2.6/site-packages/twisted/internet/defer.py", line 1045, in _inlineCallbacks
            result = g.send(result)
          File "/var/praekelt/vumi-go/ve/src/vumi/vumi/components/window_manager.py", line 110, in get_next_key
            yield self._set_timestamp(window_id, next_key)
          File "/var/praekelt/vumi-go/ve/src/vumi/vumi/components/window_manager.py", line 115, in _set_timestamp
            flight_key: self.get_clocktime(),
        exceptions.TypeError: zadd() keywords must be strings
```