Migrate to key-value store & improve persistence reliability

[kincaidoneil]

Pros:

• Atomic writes, rather than triggered on a timer (no reliance on a graceful disconnect to ensure everything is written)
• More reliable cross-platform support
• Compatible with the existing store model in plugins, which simplifies the implementation
• If only the values are encrypted (and not keys), we have more data to restore lost funds if users forget their password

Cons:

• Single JSON file provides a more portable config across machines
• Some DBs are a pretty heavy dependency, and use native modules that can cause problems with Electron
• If only the values are encrypted, the keys can expose metadata (likely not a big deal, the pros of this I definitely think outweigh the cons)

My gut feeling is we'll need to implement our own encryption layer for either JSON or a key-value store, since at least for LevelDB/level, it didn't look like there were many great options.

@rhuairahrighairidh Thoughts/anything else?