Make IConnection single-use and move restart/reuse behavior to HubConnection

[analogrelay]

It makes things a little cleaner and after all, the main reason we want restartable/reusable connections are to keep HubConnection bindings in place, so making HttpConnection/IConnection restartable is less important.

[davidfowl]

So thinking about this more in the context of my existing change, we might be able to just unify things like this:

ConnectionContext is the root of the universe, its used on both the client and the server as the connection abstraction. We remove IConnection completely and introduce a factory of some sort that looks something like this:

public interface IConnectionFactory
{
    public ValueTask<ConnectionContext> ConnectAsync();
}

We then turn the HttpConnection into the HttpConnectionFactory whose solve purpose is to provide the ConnectionContext implementation over the various http transports.

I'll work through it.

[davidfowl]

Closing in favor of #1450

[davidfowl]

Closing in favor of #1475