Tune indexing performance

[glennhickey]

There are some questions that can be answered empirically

• What kind of interval do we want between anchor lines ?
• How big should index blocks be (also, they should probably be explicitly be defined on bytes rather than reference bp)?
• Can we make the index offsets in uncompressed space (I had to switch to bgzip-native coordinates to get it working, but hopefully there's a way to not need this)?
• Should .tai files be bgzipped by default by taffy index (probably yes, easy to test)