I think we should likely GC old notifications. While GC'ing feed stories is more arguable and I could go either way on it, notifications seem useless after a fairly short time window, and this table can get huge for active installs. We could do something like GC after 90 days safely, I think.
To do this, we'll need to add a `chronologicalKey` key to the table.
Even on this install, which is relatively small in terms of potential to generate these rows, we have 400K of them and 150MB of data:
```
mysql> show table status like 'feed_storynotification';
+------------------------+--------+---------+------------+--------+----------------+-------------+-----------------+--------------+------------+----------------+---------------------+-------------+------------+-------------+----------+----------------+---------+
| Name | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment |
+------------------------+--------+---------+------------+--------+----------------+-------------+-----------------+--------------+------------+----------------+---------------------+-------------+------------+-------------+----------+----------------+---------+
| feed_storynotification | InnoDB | 10 | Compact | 397141 | 139 | 55279616 | 0 | 101302272 | 6262095872 | NULL | 2015-06-08 17:51:12 | NULL | NULL | utf8mb4_bin | NULL | | |
+------------------------+--------+---------+------------+--------+----------------+-------------+-----------------+--------------+------------+----------------+---------------------+-------------+------------+-------------+----------+----------------+---------+
1 row in set (0.09 sec)
```
```
$ ./bin/storage probe
Analyzing table sizes (this may take a moment)...
APPROXIMATE TABLE SIZES
...
phabricator_feed 309.3 MB 4.3%
feed_storyreference 70.3 MB 1.0%
feed_storydata 89.7 MB 1.3%
feed_storynotification 149.3 MB 2.1%
...
```
This is the largest log-esque table that we don't currently GC.