Be even more explicit about the rules for holding mem.mutex
Be even more explicit about the rules for holding mem.mutex

Document that field accesses must be done with the mutex held,
and rename uncompressedDigest to uncompressedDigestLocked.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image