Improve reclaim

@thieman This is motivated by my strong suspicion that these intermittent random stat queue response time spikes are basically due to reclaims of unfinished items which were in progress when a worker died. Right now these items will sit around and only get reclaimed when a new worker starts up. 

So I've done the following:
* Actually fire the `on_reclaim` event handler when we reclaim (must've been accidentally removed previously) so we have data on how often we do this.
* Run reclaim every 50 heartbeats rather than every time we start so that we don't allow items to fester for hours.
* Reset `item['ts']` when reclaiming so that it doesn't cause an artificial response time spike.