Merge pull request #2842 from fjl/downloader-remove-eth61

eth/downloader/downloader.go

eth/downloader/downloader_test.go

eth/downloader/metrics.go

eth/downloader/peer.go

eth/downloader/queue.go

eth/downloader/types.go

eth/fetcher/fetcher.go

eth/fetcher/fetcher_test.go

eth/fetcher/metrics.go

eth/handler.go

eth/handler_test.go

eth/metrics.go

eth/peer.go

eth/protocol.go

eth/protocol_test.go