align checkpoints with BGZF block boundaries

[mlin]

This would facilitate zero-transcoding htsget on spvcf.gz. Would need to use htslib BGZF primitives directly inside spvcf instead of piping into bgzip.

Fallback idea: add a flag to the decoder directing it to ignore any input rows preceding a checkpoint. Still, for the zero-transcoding case we need a way to guarantee we include the previous checkpoint.