Investigate performance impact of only triggering Mirror updates if a record value has actually changed

[gavin-norman-sociomantic]

There are sometimes cases where a Put request will overwrite a record with the same value (i.e. unchanged). These cases will currently trigger Mirror and Listen updates unnecessarily, leading to wasted resources in the DHT, network, and mirroring applications.

It would be worth testing the performance impact of checking if the record value has changed before triggering listeners. (It may turn out that the impact of this change would be greater than the impact of not doing this check, but it seems worth looking into quickly.)