Merge changes from topic "CowXorOp"

* changes:
  libsnapshot: Don't PrepMergeOps on resume
  snapuserd: Add support for Xor ops in snapuserd
  snapuserd: Rename Read Ahead Iterator
  snapuserd: Add XorSink
  libsnapshot: Clone worker readers from snapuserd
  libsnapshot: Add support for Xor ops in Cow Format