Stream chunks when sending chunked files

Summary: Ref T7149. Return a real iterator from the Chunk engine, which processes chunks sequentially.

Test Plan:
This is a bit hard to read, but shows the underlying chunks being accessed one at a time and only some being accessed when requesting a range of a file:

```
$ ./bin/files cat F878 --trace --begin 100 --end 256
...
>>> [10] <query> SELECT * FROM `file_storageblob` WHERE `id` = 85
<<< [10] <query> 240 us
 better software.

Phabricat>>> [11] <query> SELECT * FROM `file_storageblob` WHERE `id` = 84
<<< [11] <query> 205 us
or includes applications for:

 >>> [12] <query> SELECT * FROM `file_storageblob` WHERE `id` = 83
<<< [12] <query> 226 us
 - reviewing and auditing source>>> [13] <query> SELECT * FROM `file_storageblob` WHERE `id` = 82
<<< [13] <query> 203 us
 code;
  - hosting and browsing >>> [14] <query> SELECT * FROM `file_storageblob` WHERE `id` = 81
<<< [14] <query> 231 us
repositories;
  - tracking bugs;
```

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: joshuaspence, epriestley

Maniphest Tasks: T7149

Differential Revision: https://secure.phabricator.com/D12073

src/applications/files/engine/PhabricatorChunkedFileStorageEngine.php

@@ -177,4 +177,15 @@ final class PhabricatorChunkedFileStorageEngine
     return 32;
   }
 
+  public function getFileDataIterator(PhabricatorFile $file, $begin, $end) {
+    $chunks = id(new PhabricatorFileChunkQuery())
+      ->setViewer(PhabricatorUser::getOmnipotentUser())
+      ->withChunkHandles(array($file->getStorageHandle()))
+      ->withByteRange($begin, $end)
+      ->needDataFiles(true)
+      ->execute();
+
+    return new PhabricatorFileChunkIterator($chunks, $begin, $end);
+  }
+
 }