Before this commit, we performed a full read-modify-write cycle on a
partition's hashes.pkl to invalidate the relevant suffix whenever an
object PUT or DELETE completes.
Now we append invalid suffixes to a new file, "hashes.invalid",
alongside hashes.pkl. When we actually get a REPLICATE request and
need to compute the hashes, *then* we perform a full read-modify-write
on hashes.pkl and clear out hashes.invalid.
Change-Id: Ia7add438e25688b4b286f0110a4e43490e11ad75
22685d6231