Misc improvements

[fillson-shady]

During a testing Coyote helped us a lot but we also faced some issues due to unreliable WiFi connection.
Once connection to a RabbitMQ cluster is interrupted subsequent messages will be lost.
Moreover upon restarting Coyote wipes an existing DB.

So I would like to contribute a couple of improvements:

• if the DB already exists then do not recreate it
• an option to make a persistent queue, so messages won't be lost if connection is interrupted
• automatically try to reconnect if connection is interrupted

[ghokun]

Changes are so nice! Is it possible to split them into different PR's? So I can rollback or revert to an older version in case there are regressions. In future I plan to extend test cases so it will not be a problem however now I do not trust my code :D

1. Append to existing DB
2. Add persistent option
3. Auto reconnect on connection loss

[fillson-shady]

Sure, I'll create 3 separate PRs then 👍 Thank you for reviewing!