Use pessimistic locking to prevent tree corruption

[romanb]

Pessimistic locking support of JPA 2 should be used to prevent tree corruption in concurrent scenarios. A lock on the root node of the tree being modified may be enough.

[JustinTArthur]

I've started some work on this in BlackLocus' Pessimistic-Locking branch, but I've only added it for the node-manipulation functions I'm using for our project at the moment. It might be a good starting point or a dead end.