Lru OnRemove locks value instead of item

[bitfaster]

Fixes a regression introduced in 2.3.0 when updating the ConcurrentLruCore.TryRemove methods.

This caused the dispose code path to fail when caching a null. Therefore, introduce a test for round tripping a null value through ConcurrentLru and ConcurrentLfu.

[coveralls]

coverage: 98.517% (+0.1%) from 98.388% when pulling 5700be0 on users/alexpeck/rembug into 4d52e6f on main.