torrents get lost

[Xuefer]

steps to reproduce:

1. make sure there're more than 128 torrents added, in any state
2. use ltconfig plugin to set alert_queue_size to 100 (or load preset: minimal memory usage, which use 100 for alert_queue_size)
3. restart deluge

only about 126~130 torrents get loaded. restart deluge again you'll see size of torrents.state get slim down to a few kbytes from mbytes "Finished loading 427 torrents in 0:00:06.760026" is not appearing in the log. unless the count is slim down

alert_queue_size = 100 is not required for this to happen

Reason: loading existing torrents, rely on alert_queue as callback Suggestion: need a reliable callback mechanic other than alert_queue which is limited by size.