If a CC takes too long (around 50 slices) or gets interrupted by a GC,
we have to finish it synchronously, which can cause a big pause.
This patch tries to avoid that by eagerly increasing the slice budget
the longer a CC goes on. It linearly increases the slice time from 5ms
to 40ms as we approach the halfway point of a CC (1 second), matching
GC pauses, and then leaves it at 40ms.