Add package level accessor methods to test factories
Extract createMockPath to a utility class
Add FileChannelProvider

* Extract common test methods to FileChannelProviders
to AbstractFileChannelProviderTest.

* Remove MockPathUtils, and move createMockPath() to
AbstractFileChannelProviderTest

* Add an additional test method to determine if a Writer is
open via reflection. Beefed up several tests to check that
Writers are open or closed.
Initial revision of AsyncPathDestination