Hub methods can soft-lock the connection

Today we detect if you allow parallel hub invocations and throw if you don't when trying to use the feature. This doesn't work if you use IHubContext in the Hub, or if you have multiple waiting results for the same connections Hubs.
This is also especially bad in OnConnectedAsync because that's a special method that runs before the receive loop starts, we need to throw/unblock/warn etc. for this