[SignalR cpp] Allow handler registrations while connection is active

[BrennanConroy]

Currently hubConnection.on(...) will throw if the connection is active, this isn't the pattern we follow in the other clients and we should allow handlers to be registered while the connection is running.

https://github.com/aspnet/SignalR-Client-Cpp/blob/bcea0940576306ff625caa508b4f6a2a80d3fbe3/src/signalrclient/hub_connection_impl.cpp#L83-L86

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[CodeMnke]

All you need to do is just remove the check, there is no reason for it.

[BrennanConroy]

All you need to do is just remove the check

Not quite, the collection being inserting into is not thread safe so more thought needs to be put into the fix than just "remove the check".

If you're passionate about this issue the repo is open source, feel free to open a PR to fix it 😃