Merge pull request #2742 from karalabe/tx-spam-protection

core/tx_list.go

core/tx_list_test.go

core/tx_pool.go

core/tx_pool_test.go

core/types/transaction.go

core/types/transaction_test.go

eth/api_backend.go

eth/handler.go

eth/helper_test.go

eth/protocol.go

eth/protocol_test.go

eth/sync.go

internal/ethapi/api.go

internal/ethapi/backend.go

miner/worker.go