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 writes to both the instance and the upstream, and they're interleaved, and they depend on one another.
• I can hard-code the query with qsprintf() but that feels like 100x worse than this.

This allows me to do this:

id(new PhabricatorExternalAccount())
  ->setForcedConnnection($instance_conn)
  ->...
  ->save();

...and get a write to the instance database, which is at least not completely a minefield.

Test Plan: Backfilled instance accounts and got interleaved instance and upstream writes as expected.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7522

Differential Revision: https://secure.phabricator.com/D12098