HONEYCOMB-431: delegate DataModification.validate to WriterRegistry

HONEYCOMB-431: delegate DataModification.validate to WriterRegistry

This patch introduces ModifiableDataTreeDelegator.validateCandidate
that translates DataTreeCandidate to DataObjectUpdates
and delegates validation to WriterRegistry (similarly as for bulk update).

ModifiableDataTreeManager.commit implementation
invokes validation before bulk update.
To make it efficient, DataObjectUpdates are computed once
and stored in DataTreeContext.

Change-Id: If4bd558e64ed84c11c9c50c7a98a2aaa8db841bb
Signed-off-by: Marek Gradzki <mgradzki@cisco.com>