pull: Actually mmap summary files
pull: Actually mmap summary files

The change in cbf1aca1d5c08d2f40832d16670484ba878d95fb actually
only mmaps the signature file, not the summary. This change makes
use mmap both, as well as extract the cache loading into a helper
function that we will later use in more places.

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree