move util.Key into its own package under blocks

blocks/blocks.go

blocks/blockstore/blockstore.go

blocks/blockstore/blockstore_test.go

blocks/blockstore/write_cache.go

blocks/key/key.go

blocks/key/key_set.go

blocks/key/key_test.go

blocks/set/dbset.go

blocks/set/set.go

blockservice/blocks_test.go

blockservice/blockservice.go

blockservice/worker/worker.go

cmd/ipfs/init.go

core/commands/block.go

core/commands/dht.go

core/commands/pin.go

core/commands/publish.go

core/commands/refs.go

core/corehttp/gateway_handler.go

core/corerepo/gc.go