Piwik supports sending an event value, which we can use to indicate
cardinality of UISIs to be tracked instead of tracking them individually.
This means we can track them at a lower frequency of (fairly arbitrary)
60s.
Instead of pinging Analytics once per failed decryption, add the failure
to a list of failures and after a grace period, add it to a FIFO for
tracking. On an interval, track a single failure from the FIFO.
Luke Barnard