Provide an atomic getOrCreateDocument method

The idea is to provide a pattern which to get a document if it already exists or create and get the document of it does not (according to a PathRef for instance), in an atomic cluster-safe manner.

This will be useful for use case such as:

• UserWorkspaceRoot creation. Currently with concurrent requests at first access, we can possibly end up with more than one UserWorkspaceRoot. i.e. 2 documents with the same path /default-domain/UserWorkspaceRoot which relentlessly leads to duplicated user workspace for each user
• Direct results are: 2 different Favorites document, unability to fetch/retrieve existing user collections, etc.

The pattern will internally need to use transaction management, and cluster-wide locking (through the key/value service compareAndSet, which is atomic).

The following algorithm will be used:

1. a lock key is chosen to be unique based on what needs to be created, for instance repository name + parent id + child name — this can be abstracted into a lambda for genericity
2. set the lock; if locking fails then retry a few times with exponential backoff and on failure return ConcurrentUpdateException
3. commit transaction + start new transaction
4. critical section: check if the doc exists; if not, create it — this can be abstracted into a lambda for genericity
5. commit transaction + start new transaction
6. remove the lock