CompositeDataStore - ReadOnly/ReadWrite Delegate Support

Support a specific composite data store use case, which is the following:

• One instance uses no composite data store, but instead is using a single standard Oak data store (e.g. FileDataStore)
• Another instance is created by snapshotting the first instance node store, and then uses a composite data store to refer to the first instance's data store read-only, and refers to a second data store as a writable data store

One way this can be used is in creating a test or staging instance from a production instance. At creation, the test instance will look like production, but any changes made to the test instance do not affect production. The test instance can be quickly created from production by cloning only the node store, and not requiring a copy of all the data in the data store.