PipeBase<TMessage> should implement the Dispose Pattern correctly

[eerhardt]

The PipeBase class implements IDisposable, but doesn't implement the basic pattern correctly.

machinelearning/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs

Lines 205 to 213 in 5e08fa1

	public void Dispose()
	{
	DisposeCore();
	}
	protected virtual void DisposeCore()
	{
	IsActive = false;
	}

See https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/dispose-pattern for the basic dispose pattern.

We should follow the design guidelines here.