R1:511a8bae34eb

Optimize feed query

Summary:
This was killing us. `EXPLAIN` after:

<table>
<tr><th>id</th><th>select_type</th><th>table</th><th>type</th><th>possible_keys</th><th>key</th><th>key_len</th><th>ref</th><th>rows</th><th>Extra</th></tr>
<tr><td>1</td><td>SIMPLE</td><td>story</td><td>index</td><td>chronologicalKey</td><td>chronologicalKey</td><td>8</td><td></td><td>201</td><td> </td></tr>…