KAZOO-801: add heuristic to compaction

Use two thresholds, ratio threshold (disk/data > 1.5) and min_data_size threshold (1MB), when deciding whether to compact a db or not. Both thresholds are in the system_config db.

These thresholds should only apply to jobs related to auto-compaction or node compaction. If you ask for a specific DB to be compacted, the heuristic should be disabled for that job.