downloader: improved downloading and synchronisation

* Downloader's peers keeps track of peer's previously requested hashes
  so that we don't have to re-request
* Changed `AddBlock` to be fully synchronous

eth/downloader/queue.go

@@ -65,6 +65,9 @@ func (c *queue) get(p *peer, max int) *chunk {
 
 		return true
 	})
+	// remove hashes that have previously been fetched
+	hashes.Separate(p.requested)
+
 	// remove the fetchable hashes from hash pool
 	c.hashPool.Separate(hashes)
 	c.fetchPool.Merge(hashes)