Verify LRU stability during warmup

[bitfaster]

Previously, items were evicted as the item count approached capacity. This was not expected behavior, items should not be evicted until capacity is reached.

Eliminating the races during Cycle and Cycle warm have made this stable.

[coveralls]

coverage: 98.598%. first build
when pulling c8c77d0 on users/alexpeck/lruwarmup
into b027a68 on main.