BUG-509: remove StoreMetadataNode.children()

This patch removes the now-unneeded access, which means the children
iteration order is no longer part of StoreMetadataNode, allowing us to
use more memory-efficient HashMap instead of LinkedHashMap. Let's do
precisely that.

Change-Id: I0764bfa9861f9df4d3014151db96edf1024e3f9b
Signed-off-by: Robert Varga <rovarga@cisco.com>
BUG-509: remove DataAndMetadataSnapshot.Builder.getDataTree()

This getter and its associated field is completely unused. Remove it to
improve clarity.

Change-Id: Ibe0145bee610c7a2b147852f43d0492ca5061adc
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge changes Ibe0145be,I0764bfa9

* changes:
  BUG-509: remove DataAndMetadataSnapshot.Builder.getDataTree()
  BUG-509: remove StoreMetadataNode.children()