remove context from HasBlock, use bitswap process instead

blockservice/blockservice.go

blockservice/worker/bench/main.go

blockservice/worker/bench_worker_test.go

blockservice/worker/worker.go

blockservice/worker/worker_test.go

exchange/bitswap/bitswap.go

exchange/bitswap/bitswap_test.go

exchange/interface.go

exchange/offline/offline.go

exchange/offline/offline_test.go