Improve connectors UX

[nabokihms]

Is your feature request related to a problem?

Just some US improvements that came to my mind.

Describe the solution you'd like to see

• The local id should be reserved (or we can wait for this issue to become resolved). Using local and password db may lead to some artifacts.
• IDs must be unique according to the documentation. I assume that we need to force it in the code as well.
• We need to figure out how to save connectors to the database. There is a code for each storage implementation that allows us to do it, but no API endpoints (corresponding PR here, but it is a little stale).
• A static connector will be picked if its id is overlapped with a connector from the database. We need to warn a user if this happens.

So, the solution is:

1. local - reserved
2. Connectors must have unique ids across the database and config file
3. Warn user if ids are overlapped