Rather than having to manually use `initializedPromise`, which really ought to be a private property, to ensure that setting/getting values in the `ViewHistory` works as intended, this re-factoring simply changes all of its methods to be asynchronous.
Furthermore, a `getMultiple` method (mirroring the existing `setMultiple` one) is also added to `ViewHistory`.
Finally, this patch also addresses an existing issue, where certain preferences (e.g. the default zoom level) would be ignored when calling `setInitialView` if reading from the `ViewHistory` fails for some reason.
Note that as discussed on IRC, this makes the viewer slightly slower to load *only* in `gulp server` mode, however the difference seem slight enough that I think it will be fine.
For consistency, I also renamed the `FIREFOX/MOZCENTRAL` sessionStorage key, but given that sessionStorage is a lot less permanent than localStorage it didn't seem necessary to migrate any existing values.
Fixes 7760.
Currently this constant is present in `viewer.js`, but it is not used there at all. Instead, it is used in `view_history.js` where we have a global for it. We might as well move the constant to `view_history.js` as that is the only place where it is used, thereby removing a global and an unused constant from `viewer.js`.
