DB should be conserved between when the plugin is updated

[fleandrei]

The DB is currently stored in the same folder as the binary.
When the plugin is deleted, the db is deleted too. When the plugin is updated, all data related to staking address are lost.

Data stored in DB should be conserved when the plugin is updated.
If a user has setup staking addresses with specific target rolls, these addresses should be still there after the update.
The total value history, rolls operation history should be preserved too.