Improve Magma Eviction

When testing with Data Blocking Cache, we found there is a ~8x performance degradation. After some investigation, it is found that magma has undo one of the optimization that skip caching the leaf node of the bottom tier sstables/b-trees. After putting back that optimization, we see the performance is on-par than before without data cache blocking.

With data block caching, we see there is a 30% degradation at 10% RR. At 50% RR, we see performance gain at less than 10%.

Based on this observation, it is imperative to keep the intermediate index blocks in cache before data blocks or the leaf nodes of the bottom index tier. This means the eviction algorithm cannot keep all blocks as equal.