core: Rework archive-z mode to have header be uncompressed

This is an incompatible change to archive-z, thus it is now renamed to
archive-z2 and ostree will no longer parse archive-z.

I noticed in perf that we were spending some time zlib-decompressing
file headers, which is just inefficient.  Rather than do this, keep
the headers uncompressed, and just zlib-compress content.

src/libostree/ostree-repo.h

@@ -45,7 +45,7 @@ GFile *       ostree_repo_get_path (OstreeRepo  *self);
 typedef enum {
   OSTREE_REPO_MODE_BARE,
   OSTREE_REPO_MODE_ARCHIVE,
-  OSTREE_REPO_MODE_ARCHIVE_Z
+  OSTREE_REPO_MODE_ARCHIVE_Z2
 } OstreeRepoMode;
 
 gboolean       ostree_repo_mode_from_string (const char      *mode,