Bug 2690 - Yang-Data-Impl: write and then merge on same list in

Bug 2690 - Yang-Data-Impl: write and then merge on same list in
modification produces incorrect result

* added check for previous WRITE operation in recursiveMerge().
If there is some WRITE operation before MERGE we create new WRITE
operations for children => our data created by WRITE won't be
overwritten by merge on container/list.
* These children WRITE transactions will be recursively pushed down in the tree
while there are merge operations on them

Change-Id: Ie346c61c52cb000ee950d137f39d42e1ead3e1ff
Signed-off-by: Jan Hajnar <jhajnar@cisco.com>