Add a node "timestamp" (perhaps in the certnames table) that behaves like the other lifetime-related timestamps, i.e. the factsets/reports timestamps
Extend the configure expiration command to allow "node" as a value in the "expire" field, i.e. {"node": <boolean> ...} and have the command behave as it does for facts, but
Support a way for users to explicitly refresh the timestamp used for node TTL.
This can be accomplished currently by repeatedly sending *configure expiration* commands for a node with *expire.facts = true* (see linked ticket for more detail)
We should make this more explicit (maybe adding a new command) and look into the possibility of allowing bulk updates given a list of certnames.