Allow near-time discovery of changes in a repository

The getContentChanges operation allows external systems to find out what has changed in repository. That works well if the external system asks for the changes once in a while, for example in an interval of a few hours. It's not suitable for near-time discovery of changes.
Ideally, CMIS would provide some sort of subscription to repository changes, but I don't believe that would be supported by many repositories.
Instead, getContentChanges could be extended by a "timeout" parameter. With that the parameter the client can ask the server not return an empty list if no changes are present. Instead, the server waits and holds the connection until either a change can be reported or the timeout expires. That allows a client to get near-time updates without polling the server every few minutes or seconds.