Fix for problems with using RavenFS in embedded mode

This change makes the AsyncFilesSession use the AsyncFilesServerClient owned by its parent FilesStore instead of creating a new one that doesn't inherit the HttpJsonRequestFactory using the OwinClientHandler to send requests.

This fixes the problems I was having with getting a FIleNotFoundException when trying to store a new file because the FilesSession was trying to make requests to http://localhost/ and getting a 404 error from my local IIS.