BUG-509: cleanup StoreMetadataNode

Removes StoreTreeNode<StoreMetadataNode> interface and tightens
visibility of a few methods.

Change-Id: I3bbf303025046f217bdcea81fcc14c420dbf9478
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-509: cleanup StoreMetadataNode"