Allow splitting client to support concurrent read/publish

[Wazner]

I'm using this library from a web application used to control IoT devices. I would like to publish a message in response to a web call. The current API does not allow concurrent use of the Client.

In this PR I implemented a into_split method which, much like the tokio::net::TcpStream::into_split will split the Client into two halves: a reading half and a publishing half. The publishing half is Sync and clonable, which allows use from multiple threads. The reading half is not, and remains usable from only a single thread.

The current implementation in this PR is very rough (just copied various methods from the Client) and serves my purpose. If the maintainer of this repository is interested in this functionality, I will clean it up further.

EDIT: API design changed a bit since original design described above. See https://github.com/fluffysquirrels/mqtt-async-client-rs/pull/27#issuecomment-923872718

[fluffysquirrels]

I think this is an interesting feature request. Unfortunately I have issues with the way it's currently implemented in your branch. From a brief look it seems quite a lot of code is duplicated between the new ClientReadHalf and ClientPublishHalf implementations and the original Client, and I think this would be quite hard to maintain correctly.

Is there a way for the Client*Half implementations to delegate to an inner Client, or for all of Client*Half and Client to delegate to a new ClientInner that holds most of the logic?

[Wazner]

Thank you for the feedback. I'll work on it sometime this week:

• refactor to remove code duplication. Probably remove send functions from client, and make them functions accept individual parts instead of client struct.
• add better documentation, with example

[Wazner]

I have changed the API to something I am more comfortable with, removed code duplication and added some documentation.

The new API works by allowing the creation of ClientPublisher instances (similar to the PublishHalf in the earlier concept). These instances can be cloned and are Send (so they can be used from different threads).

An example how this works:

use mqtt_async_client::client::Client;

let client = ..;

let publisher1 = client.publisher();
let publisher2 = client.publisher();  // Or use publisher1.clone()

tokio::spawn(async {
  publisher1.publish(..)
});

tokio::spawn(async {
  publisher2.publish(..)
});

The original Client instance remains usable after creating one or more publishers.