R1:827c0ce08134

Allow LiskDAO to be forced to use a specific connection

Summary:
Ref T7522. This seems like the least-bad approach to a messy issue:

- When backfilling accounts from an imported instance, I need to write ExternalAccount rows to the instance to link instance accounts with upstream accounts.
- We do this in the daemons in some other cases, which lets us run all the code in the context of the instance. However, I really want to do this in-process here because it's way way simpler and we need to do…