DirectoryLayer: added debug annotations, use FullName when possible in logs, exposed the RegisterAsync method

- added the RegisterAsync() method on IFdbDirectory (register a directory with an already existing prefix)
- added global flag FdbDirectoryLayer.AnnotateTransaction (off by default) to help troubleshoot the layer
- use folder.FullName to get the formatted path
- added more XML comments
Temporary mute the unit tests on compressed bitmaps