using the cleanupTime to checkExpiration instead of cleanupInterval

[icaiyu]

I notice that let cleanupInterval to check the expiration is not that efficient.
Like this example:
Add an item with lifespan = 10 to a blank table, the cleanupInterval will be 10s,

6s later, add another item with lifespan = 8, it will call the table.expirationcheck()
because item.lifespan < cleanupInteval
in fact, it's unnecessary to check because 10 - 6 < 8 (the cleanupTimer countdown to 10-6 = 4) . Since we couldn't get the countdown time of the cleanupTimer directly, note down the cleaning time as cleanupTime is helpful to make it.

[coveralls]

Coverage remained the same at 100.0% when pulling a957994 on icaiyu:master into 46a3a44 on muesli:master.