Implement PersistenceManager for definitions stored in JCR

This provides the CRUD management for the JCR storage model (HCM-57)

The proposed API would provide these features:

1. write a MergedModel into the JCR as the baseline
2. load a MergedModel (without content definitions) from the baseline
3. check whether a MergedModel has config or content path differences relative to the stored baseline

3 should be as fast as possible to support a fast check of whether config from the classpath matches the baseline.

Note, this first implementation will not attempt to compare the baseline with active configuration as stored in the JCR. See HCM-78 for that feature.