R1:c19bb57730a2

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>…