Feature: create indexes within a track database for JBrowse tracks without sufficient indexes

[nathandunn]

What we want to do here is emulate the behavior we get for NCList backed GFF3 (json) tracks or objects with a combined density track.

The major reasons behind this are:

• we can use GFF3 tracks (tabixed) without converting to JSON, we will create an index of the track and store it directly in the database to pull out to create histograms
• we can do the same for vcf tabixed data
• we can do the same for index BAM data

[nathandunn]

Its possible that a summary index is built on the track itself, in which case this won't be necessary at the track level. However, may not be a priority at this point.