Add pagination support to the getChildren() call, allowing clients to iterate over children N at the time.
Motivations for this include:
- Getting out of a situation where so many children were created that listing them exceeded the network buffer sizes (making it impossible to recover by deleting)
- More efficient traversal of nodes with large number of children 
I do have a patch (for 3.4.6) we've been using successfully for a while, but I suspect much more work is needed for this to be accepted.