pull: use a single per-transaction syncfs instead of fsync

Do not write directly to objects/ but maintain pulled files under tmp/
with a "tmpobject-$CHECKSUM.$OBJTYPE" name until they are syncfs'ed to
disk.

Move them under objects/ at ostree_repo_commit_transaction cleanup
time.

Before (test done on a local network):

$ LANG=C sudo time ./ostree --repo=repo pull origin master

0 metadata, 3 content objects fetched; 83820 KiB; 4 delta parts
fetched, transferred in 417 seconds
16.42user 6.73system 6:57.19elapsed 5%CPU (0avgtext+0avgdata
248428maxresident)k
24inputs+794472outputs (0major+233968minor)pagefaults 0swaps

After:

$ LANG=C sudo time ./ostree --repo=repo pull origin master

0 metadata, 3 content objects fetched; 83820 KiB; 4 delta parts
fetched, transferred in 9 seconds
14.70user 2.87system 0:09.99elapsed 175%CPU (0avgtext+0avgdata
256168maxresident)k
0inputs+794472outputs (0major+164333minor)pagefaults 0swaps

https://bugzilla.gnome.org/show_bug.cgi?id=728065

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

src/libostree/ostree-repo-private.h

@@ -34,6 +34,8 @@ G_BEGIN_DECLS
 struct OstreeRepo {
   GObject parent;
 
+  char *boot_id;
+
   GFile *repodir;
   GFile *tmp_dir;
   int    tmp_dir_fd;
@@ -82,6 +84,11 @@ _ostree_repo_ensure_loose_objdir_at (int             dfd,
                                      const char     *loose_path,
                                      GCancellable   *cancellable,
                                      GError        **error);
+void
+_ostree_repo_get_tmpobject_path (OstreeRepo       *repo,
+                                 char             *output,
+                                 const char       *checksum,
+                                 OstreeObjectType  objtype);
 
 gboolean
 _ostree_repo_find_object (OstreeRepo           *self,
@@ -101,6 +108,7 @@ _ostree_repo_has_loose_object (OstreeRepo           *self,
                                OstreeObjectType      objtype,
                                gboolean             *out_is_stored,
                                char                 *loose_path_buf,
+                               GFile               **out_stored_path,
                                GCancellable         *cancellable,
                                GError             **error);