Allow schema changes

The platform must allow ways of changing schemas during the development of a project without having to delete all the storage or break all existing documents.

Use cases:
1. change a field from String to Date (for instance),
2. remove a field,
3. remove a schema,
4. (the use case of adding a field or schema is treated by the simpler NXP-237)

Note that any solution will have to take into account migration inside the version storage, which is mostly immutable in JCR implementations.