cache ostree -> composefs metadata

[cgwalters]

After #3326 we're now back to the situation where creating a deployment is O(metadata) - times two (once for the hardlink farm, once for composefs). Here with a nontrivial host each takes about 1.5-2s which isn't fatal or anything but is definitely noticeable.

For making the composefs it seems we're just getting hit by syscall overhead, especially for listing xattrs. But we also have the open() + ioctl() + close() for the verity digest.

We could significantly cut down all of this with what I think would be a simple hack - if we're booted from a composefs, then read it into memory and create a hash table mapping ostree checksum ➡️ composefs node, and use it as a lookaside in ostree_repo_checkout_composefs().