Cache whether a head request succeeds for a given URL
Cache whether a head request succeeds for a given URL

When loading alignments we often making multiple requests in succession, it's time consuming to have to try HEAD for each one

amwenger/igv,amwenger/igv,godotgildor/igv,godotgildor/igv,amwenger/igv,amwenger/igv,itenente/igv,igvteam/igv,amwenger/igv,igvteam/igv,godotgildor/igv,godotgildor/igv,igvteam/igv,itenente/igv,igvteam/igv,itenente/igv,itenente/igv,itenente/igv,igvteam/igv,godotgildor/igv