When parent node with child nodes is written, read of child node using InstanceIdentifier returns null

Steps to reproduce:

1. Start transaction
2. Inside transaction create a Flow-capable table which contains one flow
3. Commit transaction
4. Invoke read operation with Instance Identifier directly pointing to the flow

Step four should return data for written flow, but returns null instead.
This is reproducible for all access layers such as Restconf, Binding Independent Broker and Binding Aware Broker. so the bug is in underlaying data store (SchemaAwareDataStoreWrapper and HashMapDataStore).

Regression test suite is attached.