* *: Remove usage of custom buffer initialization usage
With version `0.3.0-alpha.19` the futures-preview crate makes the
`AsyncRead::initializer` API unstable.
In order to improve interoperability with e.g. both a library depending
on alpha.18 as well as a library depending on alpha.19 and in order for
rust-libp2p to become stable again, this commit removes all usages of
the unstable `initializer` API.
* protocols/noise: Remove NoiseOutput Asyncread initializer
* transports/tcp: Remove TcpTransStream AsyncRead initializer
* *: Remove version pinning of futures-preview to 0.3.0-alpha.18
With version 0.3.0-alpha.19 the futures-preview crate makes the
AsyncRead::initializer API unstable. Given that the previous commits
removed usage of the initializer API, the version pinning is not needed
any longer.
