Don't store updated time on faults.

[jimfulton]

The updated time doesn't actually reflect when the test was done. Just using when we called set_faults is enough.

[freddrake]

Currently, for the DynamoDB data store, updated is only stored on heartbeat records (where it's appropriate).

The MemoryDB does store the updated time, but that seems acceptable since it doesn't have the cost implication as it does for the DynamoDB storage.

Is anything further needed here?