Garbage collect old configs

[jml]

From @jml on January 24, 2017 18:28

We currently append a new row for each config. We should GC the old ones so the database doesn't get super full, especially since automated processes push configs.

Copied from original issue: weaveworks/service#1082

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 30 days. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.