Set priority of queue'd items to 9

[dokterbob]

As is done in ipfs-search add. Otherwise, items will most likely never be crawled; they will simply drop off the queue.

Ref:

• https://github.com/ipfs-search/ipfs-search/blob/master/commands/add.go#L65
• https://www.rabbitmq.com/priority.html

Related: https://github.com/ipfs-search/ipfs-search/issues/243