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

[artemredkin]

I we add method with the same name but different type we will break compilation, because some calls could become ambiguous 😢

[Lukasa]

Given that there is discussion about possibly breaking API in the nearish future anyway, we may find we want to delay this until then to resolve this problem.

[Lukasa]

Is there a reason we're writing this as a callback-taking type rather than just holding onto the task object?

[artemredkin]

and "forward" all write requests to it's channel? that might be a good idea, will try that, thanks!

[Lukasa]

Given that there is discussion about possibly breaking API in the nearish future anyway, we may find we want to delay this until then to resolve this problem.

[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.