Manage removal of refs

Implement the apply-object API where the revision data
is null, interpreting the call as a ref delete.
The approach is taken because of the need of backward compatiblity
with existing REST-API on the stable-3.3 branch.

This fixes the problem of the lack of replication for
delete changes.

Bug: Issue 15618
Change-Id: I305ebcae3e997f64c8d2f5f452114f5d771399b5