Storage provides the flush operation.

A capability to flushing the storage is needed by a client such as
C3PODataManager. This adds flush():Unit to Storage interface and
provides just placeholders for actual implementation to come later.
with ZookeeperObjectMapper, InMemoryStorage, and
ZoomStorageServiceTestUtils.

Fix MN-2999
Change-Id: I0619dab28089dc424b41acb105e87c51b25e8562
Merge "Storage provides the flush operation."