core: Add dynamic uncompressed object cache for archive-z

This gives us something closer to the advantages of archive and
archive-z when using the latter.  Concretely we get deduplication
among multiple checkouts, along with the "devino" hash table trick
during commits to avoid checksumming content again.

This is enabled by default.

src/libostree/ostree-repo.h

@@ -281,6 +281,10 @@ ostree_repo_checkout_tree_finish (OstreeRepo               *self,
                                   GAsyncResult             *result,
                                   GError                  **error);
 
+gboolean       ostree_repo_checkout_gc (OstreeRepo        *self,
+                                        GCancellable      *cancellable,
+                                        GError           **error);
+
 gboolean       ostree_repo_read_commit (OstreeRepo *self,
                                         const char *rev,
                                         GFile       **out_root,