Users can set number of items, but that's hard to tune without knowing a lot of internal details about how large metadata is. Easier to instead set the size in bytes.

We already require impl DeepSizeOf, so it should be very little effort to make eviction size based.

We should have a deprecation cycle for the old item-based size. It can assume a fixed-size entry (2MB?) and use that to derive a value for the max bytes.