refactor ObjectManager to have just one node array

[dbu]

the node array is currently nested, having one list per class (Node, Version, VersionHistory). this makes no sense as one tree element is always of exactly one of those types. instead of passing the class to build in, we should automatically determine that from the primary type of the node. right now if i getNode() a version node i would get a plain node for that which makes no sense.

also Version and VersionHistory are immutable, except through the VersionManager. we should enforce that to avoid wtf errors on save.