tabix slicing improvements

[mlin]

1. Current logic extracts records with POS >= qlo (query low) but omits records with POS < qlo && END >= qlo.
2. More thorough testing would be nice.
3. The way checkpoints are detected feels vaguely brittle.

[mlin]

It would help to require that a checkpoint record must not follow any other record whose END >= checkpointPOS (on the same contig)