SQLite support

[a-type]

Starting to look into React Native... or even just Electron-esque things.

Work required...

• Refactor persistence layer into abstractions (still IDB only)
  • Refactor metadata persistence
  • Refactor Metadata class to use persistence layer
  • Refactor metadata migrations
  • Refactor queryable persistence
  • Refactor queries to utilize persistence layer
  • Refactor queryable migrations
• Implement persistence abstractions in SQLite
  • Metadata
  • Document indexes
  • Files (with, I guess, local filestorage)

[a-type]

Mini-todo: refactor WIP to simplify persistence impl:

• Make WIP change namespace during client init
• Add "copy namespace to namespace" to persistence impls
• Add "list all namespaces" to persistence impls
• Rework WIP to be "copy default_ns to wip_ns, then open wip_ns"
• Rework import to be "load data to temp_ns, then copy to default_ns"

[a-type]

For SQLite import/WIP copy, procedure is roughly...

• Open original namespace at version N
• Open target namespace at version N (migrate to N if needed)
• Copy database file from o->t
• Migrate target namespace to version M (if M!=N)