eth/downloader: prioritize block fetch based on chain position, cap memory use

Godeps/Godeps.json

...n/karalabe/cookiejar.v2/collections/prque/example_test.go

...gopkg.in/karalabe/cookiejar.v2/collections/prque/prque.go

....in/karalabe/cookiejar.v2/collections/prque/prque_test.go

...opkg.in/karalabe/cookiejar.v2/collections/prque/sstack.go

...in/karalabe/cookiejar.v2/collections/prque/sstack_test.go

eth/downloader/downloader.go

eth/downloader/downloader_test.go

eth/downloader/peer.go

eth/downloader/queue.go

eth/downloader/queue_test.go