* protocols/kad: Do not attempt to store expired record in record store
`Kademlia::record_received` calculates the expiration time of a record
before inserting it into the record store. Instead of inserting the
record into the record store in any case, with this patch the record is
only inserted if it is not expired. If the record is expired a
`KademliaHandlerIn::Reset` for the given (sub) stream is triggered.
This would serve as a tiny defense mechanism against an attacker trying
to fill a node's record store with expired records before the record
store's clean up procedure removes the records.
* protocols/kad: Send regular ack when record discarded due to expiration
With this commit the remote receives a
[`KademliaHandlerIn::PutRecordRes`] even in the case where the record is
discarded due to being expired. Given that the remote sent the local
node a [`KademliaHandlerEvent::PutRecord`] request, the remote perceives
the local node as one node among the k closest nodes to the target.
Returning a [`KademliaHandlerIn::Reset`] instead of an
[`KademliaHandlerIn::PutRecordRes`] to have the remote try another node
would only result in the remote node to contact an even more distant
node. In addition returning [`KademliaHandlerIn::PutRecordRes`] does not
reveal any internal information to a possibly malicious remote node.
* protocols/kad/src/behaviour: Use `now` and reword expiration comment
Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>
