4.x: Add the IConnectableObservable.AutoConnect() operator

[akarnokd]

This PR adds the AutoConnect operator as an extension method on IConnectableObservable. The operator connects the source once the specified number of observers have subscribed to it.

This operator enables caching of items via Replay() without the need to call Connect() manually and avoid reconnection which would happen with RefCount():

var cached = source.Replay().AutoConnect(2);

// connection happens only now
cached.Subscribe(Console.WriteLine);

// connection happens only now
cached.Subscribe(Console.WriteLine);

// some time later when the source has completed on its own
// this will not trigger a new connection
cached.Subscribe(Console.WriteLine);

Subscribers disposed won't dispose the connection. In case the connection itself should be disposed, it is made available via the onConnect callback:

SingleAssingmentDisposable sad = new SingleAssignmentDisposable();

var cached = source.Replay().AutoConnect(1, d => sad.Disposable = d);

var d = source.Subscribe(Console.WriteLine);

// sometime later
// it would be recommended to dispose any observers because 
// they would stay without any further events and never terminate
d.Dispose();

// stop the source
sad.Dispose();

After this, the cache cannot be resurrected and a new chain has to be created again off of source.