implement new streamwriter api

[artemredkin]

🚧 This is slightly WIP. Revamps StreamWriter.

Motivation: There are two issues with current StreamWriter API:

1. no access to allocator
2. writer completion is super confusing, it will be considered finished when the returned EventLoopFuture is completed, which is not ideal.

Modifications:

1. Deprecate old StreamWriter API
2. Introduce new type StreamWriter2 (really need help with the naming here) with the following API:

let body: HTTPClient.Body = .stream2(length: 8) { writer in
    writer.write(writer.allocator.buffer(string: "1234").whenComplete { _ in
        writer.write(writer.allocator.buffer(string: "1234").whenComplete { _ in
            writer.end()
        }
    }
}

Result: Closes #194 Closes #264

[ktoso]

Just as a heads up, the main development branch has been changed to main, following the Swift policy on this.

This PR has been re-targeted to main and should just work. However when performing rebases etc please keep this in mind -- you may want to fetch the main branch and rebase onto the main branch from now on since master is not up-to-date anymore and is going to be deleted shortly.