Match the API so we have a mock to work with and test against while its being built.
One of the ideas:

• Make ChunkManager constructor to take ClusterTime and so the instance of the ChunkManager will contain routing only <= the provided time.
• Add a method getTime() that will return the latest ClusterTime persisited i.e. used in a versioned data of the chunks that are used to create this manager. This time is <= then the provided to the c-tor time