The fleet table has two time columns and staleness only looked at one. A PC can
go quiet in either direction: silent to the server, or still posting while its
own cycle has stopped advancing. Only the first was caught, so a machine whose
Last check-in had not moved since the previous morning still showed 'ok'.
The two are tested independently rather than by taking the newer of them.
receivedat is the server's own record and cannot be argued with; lastcheckin is
asserted by the client, so a wrong clock there must not be able to vouch for a
PC the server has not actually heard from. Checking them separately means a
future-dated lastcheckin cannot mask real silence.
The tooltip now names both times, since which one stopped says what kind of
failure it is.
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