hlr_vty_subscr: prettier output for last LU seen

Extend the "last LU seen on ..." line with the amount of seconds that
passed since now, or "(invalid timestamp)".

Patch split from Id7fc50567211a0870ac0524f6dee94d4513781ba, because it
depends on timestamp_age which was just added in
Ife4a61d71926d08f310a1aeed9d9f1974f64178b.

Change-Id: I24f9e86c1aa0b1576290094e024562f41b988f37
1 file changed