Skip LFU rescheduling when expiry hasn't changed

[bitfaster]

The reschedule code is effectively a few reference assignments and simple arithmetic. However, if the time hasn't changed, we can skip running this code.

This is approximately a 4% throughput increase in this test.

[coveralls]

coverage: 99.005% (-0.07%) from 99.073%
when pulling 6e9fa58 on users/alexpeck/tlfuchk
into fa1c0fb on main.