core: Drop "staged" state for objects, refactor pull to split metadata/data

Previously we had the "staged" state to ensure we didn't add a commit
object without the associated dirtree, etc.  However it's
easier/better to just ensure in the pull command that we have all
referenced objects.

Also change pull to download metadata first.  This will allow adding
a progress bar later.

src/libostree/README.md

@@ -44,6 +44,8 @@ MILESTONE 2
 -----------
 * Store checksums as ay
 * Drop version/metadata from tree/dirmeta objects
+* Add index size to superindex, pack size to index
+  - So pull can calculate how much we need to download
 * Split pack files into metadata/data
 * Restructure repository so that links can be generated as a cache;
   i.e. objects/raw, pack files are now the canonical