core, eth, trie: direct state trie synchronization

core/blockchain.go

core/state/sync.go

core/state/sync_test.go

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/handler.go

eth/peer.go

trie/sync.go

trie/sync_test.go