install/uninstall addons in parallel

[torkus]

• [x] addons installed in parallel
  • installing and uninstalling was done sequentially to avoid Weirdness with mutual dependencies overwriting each other
• [x] acquire locks on new directories to be created (installed)
• [x] acquire locks on directories to be removed (uninstalled)
• [x] GUI updates row status individually rather than wait for all to be complete before refreshing
• [x] bug, we're losing the grouped addon drop down after an update
• [x] review
• [x] update CHANGELOG