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

src/infrastructure/storage/lisk/LiskDAO.php

@@ -188,6 +188,7 @@ abstract class LiskDAO {
   private static $transactionIsolationLevel = 0;
 
   private $ephemeral = false;
+  private $forcedConnection;
 
   private static $connections       = array();
 
@@ -280,6 +281,21 @@ abstract class LiskDAO {
   }
 
 
+  /**
+   * Force an object to use a specific connection.
+   *
+   * This overrides all connection management and forces the object to use
+   * a specific connection when interacting with the database.
+   *
+   * @param AphrontDatabaseConnection Connection to force this object to use.
+   * @task conn
+   */
+  public function setForcedConnection(AphrontDatabaseConnection $connection) {
+    $this->forcedConnection = $connection;
+    return $this;
+  }
+
+
 /* -(  Configuring Lisk  )--------------------------------------------------- */
 
 
@@ -941,6 +957,10 @@ abstract class LiskDAO {
       throw new Exception("Unknown mode '{$mode}', should be 'r' or 'w'.");
     }
 
+    if ($this->forcedConnection) {
+      return $this->forcedConnection;
+    }
+
     if (self::shouldIsolateAllLiskEffectsToCurrentProcess()) {
       $mode = 'isolate-'.$mode;