Fix fingerprint generator for new lib/pq

[SuperQ]

What did you do?

Try to update github.com/lib/pq

What did you expect to see?

Update with no issues

What did you see instead? Under which circumstances?

Deprecated: directly passing an URL to sql.Open("postgres", "postgres://...") now works, and calling this manually is no longer required.

In order to work around this we need to rework the server label name generator. Or perhaps we need to fully remove the multi-server support to remove the server label from use.