Split guessing fails with long reads

[lbergelson]

One of our developers noticed that using disq to read bams with VERY long reads (some as long as 90 megabases), is failing. Tools report success but drop all or most of the reads in the file silently.

After some investigation we've found that using a splitting index with granularity 1 fixes the problem. So it seems like there's some issue with split guessing failing on long reads.