null check for sequence tile

Fix NPE error when restoring session

Possible fix for Windows 10 genome download problem (rename fails).

Fixes igvteam#463

Remove "retrieve network" button

Fixes igvteam#452 Sashimi Plot

Synchonize queries to htsjdk SAMReader -- its not thread safe

Synchonize queries to htsjdk SAMReader -- its not thread safe

Restore default "color by" alignment setting

Present all server genomes for download, including genomes already do…

…wnloaded.