Skip Conduit call log writes in read-only mode, allowing "conduit.ping" to run

Summary: Ref T10769. See PHI8. We have an unconditional logging write which we can skip in read-only mode.

Test Plan:
  - Put Phabricator in read-only mode with `cluster.read-only`.
  - Called `conduit.ping` via web UI.
    - Before: write-on-read-only exception.
    - After: good result.

Reviewers: chad, amckinley

Reviewed By: chad

Maniphest Tasks: T10769

Differential Revision: https://secure.phabricator.com/D18233

src/applications/conduit/controller/PhabricatorConduitAPIController.php

@@ -119,9 +119,11 @@ final class PhabricatorConduitAPIController
       ->setError((string)$error_code)
       ->setDuration(1000000 * ($time_end - $time_start));
 
-    $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
-    $log->save();
-    unset($unguarded);
+    if (!PhabricatorEnv::isReadOnly()) {
+      $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
+      $log->save();
+      unset($unguarded);
+    }
 
     $response = id(new ConduitAPIResponse())
       ->setResult($result)