v3: design API to avoid allocations in responses

[oleg-jukovec]

The memory profile for a benchmark could be viewed as follows:

$ go test -v . -bench BenchmarkSync_naive_with_custom_type -benchmem -run foobar -memprofile mem.out
$ go tool pprof -http ":8080" mem.out

As you can see, several allocations are related to a response:

Our task is to design API of responses/futures in such a way as to avoid these allocations.