levels.go
func (s *levelsController) isLevel0Compactable() bool {
	return s.levels[0].numTables() >= s.kv.opt.NumLevelZeroTables
}
func (s *levelsController) isLevel0Compactable() bool {
	return s.levels[0].numTables() >= s.kv.opt.NumLevelZeroTables
}
// Returns true if level zero may be compacted, without accounting for compactions that already
// might be happening.