crawls.
* requests are serialized (using marshal by default) and stored on disk, using
one queue per priority
* request priorities must be integers now
* breadh-first and depth-first crawling orders can now be configured
through a new DEPTH_PRIORITY setting (see doc). backwards compatilibty with
SCHEDULER_ORDER was kept.
* requests that can't be serialized (for example, non serializable callbacks)
are always kept in memory queues
* adapted crawl spider to work with persitent scheduler
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