Use SQLite to cache resources

[kkaefer]

Implement part of #446:

SQLite-related tasks:

• Store expiration date
• Store resource type
• Perform conditional GETs if we already have the data, but it's expired
• Update tiles after their expiration date (first, show old tile, load new tile (if network available), store it in DB, and update the displayed tile)
• Make querying asynchronous
• Implement database size limit
• Compress data stored in database (zlib? snappy? lz4?)
• Store database in correct location (use platform:: callback to determine)
• Remove access_token from URLs before storing them in the database
• Measure SQLite query performance
• Store non-200 results
• Handle corrupt databases
• Add offline-only mode

[incanus]

Just capturing from chat with @kkaefer — we want to ensure the ability to plug in varying cache-backing files (doesn't have to be simultaneous). This way we can ensure predictability when saving "offline documents" but later updating individual tile URLs used by the normal system or other documents.