Consider mapping of XDI graph to DB records

[peacekeeper]

The term "DB record" refers to whatever is the basic unit of storage in a given DB. E.g. "row" in MySQL, "document" in MongoDB, "key/value pair" in BDB, etc.

Current mapping patterns:

• Whole XDI graph in one DB record (supported by FileWrapperGraphFactory): Perhaps best for small graphs, but not efficient for large graphs.
• One DB record per XDI context (supported by BDBJSONGraphFactory, RedisJSONGraphFactory, MongoDBJSONGraphFactory): Maybe too granular.
• One DB record per XDI statement (supported by BDBKeyValueGraphFactory, RedisKeyValueGraphFactory): Probably too granular.

Opportunity for improvement: Hybrid mapping patterns

• E.g. in general have one DB record per XDI context, but have exceptions for certain well-known XDI features.
• Link contracts should be stored in a single DB record, rather than having one DB record per XDI context or XDI statement.
• $ref/$rep relations should probably be stored in a special way, so they can be retrieved efficiently.