made GraphStageLogic.LogSource virtual and change default StageLogic LogSource

[Aaronontheweb]

Also, changed the default LogSource to be the output of the Stage.ToString() method. Right now all GraphStageLogic log sources default to just "Logic (akka://{ActorSystemName})"

[Aaronontheweb]

Here's the Shape.ToString() method implementation:

akka.net/src/core/Akka.Streams/Shape.cs

Lines 223 to 232 in 048ea84

	public bool HasSamePortsAndShapeAs(Shape shape) => Inlets.Equals(shape.Inlets) && Outlets.Equals(shape.Outlets);
	/// <summary>
	/// TBD
	/// </summary>
	/// <returns>TBD</returns>
	public object Clone() => DeepCopy();
	/// <inheritdoc/>
	public sealed override string ToString() => $"{GetType().Name}([{string.Join(", ", Inlets)}] [{string.Join(", ", Outlets)}])";

Is this too noisy for the logs? cc @ismaelhamed

[Aaronontheweb]

Decided to just log the Shape.GetType here instead - less verbose this way.

[Arkatufus]

LGTM