internal/manifest: Add L0SubLevels methods to pick compactions

This change adds methods to L0SubLevels to help pick, score,
and generate L0 -> LBase, and L0 -> L0 compactions, based on
information captured in the data structure about L0 sublevels.

These functions will be called from in compaction.go and
compaction_picker.go in a future change.

Also adds associated datadriven unit tests, and a benchmark.

Covers a large part of cockroachdb#563. Thanks Sumeer for his work, most of
this was written by him.