DataTree: write/write/merge combo may lose data

Code review of OperationWithModification.recursiveMerge() will propagate the pre-existing WRITE node's children downwards to preserve the data items which were written.

This unfortunately fails to check if there are nested nodes (introduced after the write which is being replaced), thus losing them.

The specific problem comes from the following set of operations being done on a DataTreeModification:

write(A, data1);
write(A/B, data2);
merge(A, data3);

if data1 overlaps with data2, the merge to A will push data1's version of data2 into A/B, hence the result will look as if the second write never happened.