Save syncstate

[karlb]

This PR achieves

• saving and reloading of sync states
• using separate connections each greenlet

But there's still work to do:

• [ ] Avoid database locks by splitting the data into different databases, then turn on transactions
• [ ] Clean up separation between StateDBSqlite and SQLiteStateHandler
• [ ] Proper handling of start_block for newly detected token networks

The current state can be reviewed with this in mind and merged, since it is already an improvement.

Relates to #37.