* Amend defaults
3.0.9 testing has used different defaults for cache_size and penciller_cache_size. There has been no noticeable drop in the performance of Riak using these defaults, so adopting here.
The new defaults have a lower memory requirement per vnode, which is useful as recent changes and performance test results are changing the standard recommendations for ring_size.
It is now preferred to choose larger ring_sizes by default (e.g. 256 for 6 - 12 nodes, 512 for 12 - 20 nodes, 1024 for 20 +).
By choosing larger ring sizes, the benefits of scaling up a cluster will continue to make a difference even as the node count goes beyond the recommended "correct" setting. e.g. It might be reasonable (depending on hardware choices) to grow a ring_size=512 cluster to beyond 30 nodes, yet still be relatively efficient and performant at 8 nodes.
* Comment correction
When scanning over a leveled store with a helper (e.g. segment filter and last modified date range), applying the filter will speed up the query when the block index cache is available to get_slots.
If it is not available, previously the leveled_sst did not then promote the cache after it had accessed the underlying blocks.
Now the code does this, and also when the cache has all been added, it extracts the largest last modified date so that sst files older than the passed in date can be immediately dismissed
Potentially reduce the overheads of scoring each file on every run.
The change also alters the default thresholds for compaction to favour longer runs (which will tend towards greater storage efficiency).
