Create initial federation framework and connector SPI

Create an initial implementation (or prototype) for a connector SPI and a framework for using those connectors to access content in external systems and project them into the repository content.

See the draft for a description of some of the existing components and how the federation framework might work.

This subtask includes the following capabilities:

1. Determine hooks for reading/writing in current codebase (to identify what operations we perform)
2. Create initial but basic connector SPI (e.g., "DocumentStore")
3. Modifying codebase to use any abstractions (e.g., "DocumentStore") for internal content
4. Create hard-coded connector implementation (to simplify testing)
5. Enable read-only federation (with node types already in ModeShape, no binary values, no queries, hacked configuration)
6. Initial (minimal) configuration of sources
7. Add ability to read binary values from connectors
8. Add node type support to connectors
9. Implement file system connector (read-only)
10. Enable writing (changing/adding/removing) content to the external system, perhaps without validation
11. Add ability to write binary values created by JCR clients into external system
12. Add write ability to file system connector (may be limited to no renames or moves)
13. Add support for connector-defined cache policy (e.g., cache time-to-live) on a node-by-node basis
14. Add support for connectors to have external nodes with very large numbers of children

Additional capabilities will be addressed with other subtasks on MODE-1513.