fix: persist cores

[gmaclennan]

Persist cores that are added to the core manager. Fixes #86

Todo:

• Add coreManager.close() method
• Load and open persisted cores from storage.

[sethvincent]

👍 looks cool!

The only thing I was confused by was moving the CREATE_SQL and addCoreSqlStmt to core-manager/index.js. It seemed like having them in core-index.js was a good separation. Not really a big deal though if the CoreIndex class is internal to CoreManager.

[gmaclennan]

Yeah, I did not like that either. Because events need to be added to cores loaded from persistence (which happens in the parent class) it became a very leaky abstraction to keep the persistence in the index. Couldn't figure out a neat way to separate it, but yeah, it's not pretty but it works