Sync token should be reset if IndexedDB is unavailable

[neilalexander]

Description

Safari, in a private browsing tab, just refuses access to IDB altogether. Many tears.

The sync token, which is stored in local storage, is persisted however, therefore refreshing the tab causes a /sync to take place with the last known since= but we have no IDB data to fall back on.

Steps to reproduce

• Open Safari private browsing tab
• Log into Riot
• Refresh the page
• Incremental sync takes place but IDB is gone!

We should reset the sync token back to 0 if we fail to open IDB.

Version information

• Platform: web in-browser

For the web app:

• Browser: Safari 13.0.4 in private browsing tab
• OS: macOS
• URL: All the Riots

[turt2live]

in general we should probably prevent the app from even loading

[MadLittleMods]

in general we should probably prevent the app from even loading

Seems like the app should work without IndexedDB. If it's necessary, maybe a memory store for that session although the semantics around IndexedDB would be annoying to emulate. @neilalexander's suggestion for the sync loop sounds good.

Just seems reasonable to have Element work in a private Safari browsing tab.

And seems like the same path we have to go down or in the same vein to deal with IndexedDB storage limits, #20931

[t3chguy]

We have a fallback for the IDB store which is just a MemoryStore, the issue being one of storing related data in different stores. We should move the sync tokens to the same store so if one datum dies they both die