MODE-2006 reworked "doUpdate" part of Merge operation

- doUpdate now finds correcsponding nodes by correspondingNodeForPath() which
  is based on UUIDs, instead of by name
- nodes to be removed are now removed by a new package-private internalRemove()
  that skips versioning check
- nodes to be added are now cloned instead of copied; cloning is done by a new
  package-private internalClone() that skips versioning check

Also, added a corresponding test case.