Add database garbage collector to delete expired records

[jrconlin]

Can currently be done with a cronjob that runs:

DELETE from pushboxv1 where TTL < unix_timestamp();

but it might be nice to add that to pushbox.

[edunham]

Prepping for migration:

MySQL [pushbox]> SELECT COUNT(*) from pushboxv1 where TTL < unix_timestamp();
+----------+
| COUNT(*) |
+----------+
|  1387743 |
+----------+
1 row in set (0.60 sec)

MySQL [pushbox]> SELECT COUNT(*) from pushboxv1;
+----------+
| COUNT(*) |
+----------+
|  1796240 |
+----------+
1 row in set (0.32 sec)
MySQL [pushbox]> SELECT COUNT(*) from pushboxv1 where TTL >= unix_timestamp();
+----------+
| COUNT(*) |
+----------+
|   408490 |
+----------+
1 row in set (0.62 sec)

[jrconlin]

Upping priority on this based on https://bugzilla.mozilla.org/show_bug.cgi?id=1642855

[pjenvey]

TTL lacks an index, we should also add one for this