Replace util/io with tokio

[sgkim126]

Currently, the network uses util/io. It returns a token on an event, instead of the related stream. So the network module should manage the relationship between the token and the stream. If the network module uses tokio, this complexity can be removed.

[sgkim126]

Currently, the e2e tests are flaky. It's because there can be inconsistency in the network module when creating and closing connection repeatedly. I think replacing util/io with Tokio will fix this issue.