Add pessimistic locking (lock/unlock)

We should add simple lock() and unlock() operations for pessimistic locking. A new repository capability should indicate whether the repository supports pessimistic locking or not. A new boolean property cmis:isLocked let the client know whether an object is locked or not.
The locking semantics is repository specific. For example, if locking a folder locks only the folder object or also all objects within the folder should not be covered by the spec.