Expose some DFS APIs as public or protected

Expose some DFS APIs as public or protected

Expose class DfsReader and method DfsPackFile.hasObject() as public.
Applications may want to be able to inquire about some details of
the storage of a repository. Make this possible by exposing some
simple accessor methods.

Expose method DfsObjDatabase.clearCache() as protected, allowing
implementing subclasses to dump the cache if necessary, and force
it to reload on a future request.

Change-Id: Ic592c82d45ace9f2fa5f8d7e4bacfdce96dea969