Stale records for retired sites in Memorystore

[cristinaleonr]

There are some stale entries in Memorystore belonging to retired sites. This occurred because updates for Prometheus data in Memorystore do not re(set) the key's expiration.

Example entry for retired site yul04 in production:

KEYS *yul04*
*5
$45
wehe-mlab1-yul04.mlab-oti.measurement-lab.org
$45
wehe-mlab2-yul04.mlab-oti.measurement-lab.org
$44
ndt-mlab2-yul04.mlab-oti.measurement-lab.org
$44
ndt-mlab3-yul04.mlab-oti.measurement-lab.org
$44
ndt-mlab1-yul04.mlab-oti.measurement-lab.org

HGETALL ndt-mlab2-yul04.mlab-oti.measurement-lab.org
*2
$10
Prometheus
$15

This will be fixed by #154 because Prometheus updates will only be written once entries already exist (with a corresponding expiration). That said, the existing stale entries will remain unless they are cleaned up.