Do not update last_seen value if the current message timestamp is earlier than the current last_seen time

Operations such as back-filling data can deliver messages out of order. The code currently updates the last_seen time to the message timestamp without checking if the message timestamp is earlier than the current last_seen time.

This makes it look like devices are late while the back-fill operation is running.

No file or test is named. Start by locating the handler that updates a device's last_seen value and inspect how message timestamps are compared. Done means an earlier message leaves the existing last_seen unchanged, while a later timestamp still updates it; add or run the relevant tests.

Written by the indexing model from the issue text.