Split out loose object handling from ObjectDirectory

Split out loose object handling from ObjectDirectory

The ObjectDirectory class manages the interactions for the entire object
database, this includes loose objects, packfiles, alternates, and
shallow commits. To help reduce the complexity of this class, abstract
some of the loose object specific details into a class which understands
just this, leaving the ObjectDirectory to focus more on the interactions
between the different mechanisms.

Change-Id: I39f3a74d6308f042a2a2baa57769f4acde5ba5e0
Signed-off-by: Martin Fick <mfick@codeaurora.org>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>