Revert "Fix the exit barrier in SlidingWindowReservoir."

This reverts commit 353ed4b612a734b90b56a7229a2c7ff05bb95ab2.
Revert "Switch to a two-barrier SlidingWindowReservoir."

This reverts commit 95c803d2201fa97fb2dad40944fa71baec1048e5.
Update relnotes.