Merge changes from topic "snapuserd-read-ahead"

* changes:
  libsnapshot:snapuserd:Add unit test for read-ahead code path.
  libsnapshot: Flush data to scratch space only for overlapping regions
  libsnapshot:snapuserd: read-ahead COW copy ops
  libsnapshot: Retrieve COW version from update engine manifest
  libsnapshot:snapuserd: Add 2MB scratch space in COW file
  libsnapshot:snapuserd: mmap + msync header after merge