fix perf regression when using 'owned' query

Summary:
D6335 has some unexpected side effects.  This adds back the
where clause for the owned query.  There may be other problems.

Test Plan:
Ran:

```
echo '{"query":"owned","guids":["myphid"]}' | arc --conduit-uri=https://myhost call-conduit differential.find
```

Reviewers: epriestley, dschleimer

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6832

src/applications/differential/conduit/ConduitAPI_differential_find_Method.php

@@ -69,6 +69,9 @@ final class ConduitAPI_differential_find_Method extends ConduitAPIMethod {
         $query
           ->withIDs($guids);
         break;
+      case 'owned':
+        $query->withAuthors($guids);
+        break;
       case 'phids':
         $query
           ->withPHIDs($guids);