Cache LFU drain buffers delegate

[bitfaster]

Calling drain buffers via the scheduler requires allocating an Action. This shows up when memory profiling the hit rate analysis test:

Cache the delegate to avoid this allocation. The correct approach would be for IScheduler to accept Action<T>, but that is a breaking change.

[coveralls]

coverage: 98.318% (-0.04%) from 98.36%
when pulling edeacad on users/alexpeck/lfuaction
into 0588062 on main.