Migrate to IndexedDB for improved scaling on large wallets

[sondreb]

The extension currently hold all data using the standard storage APIs on extension. We must perform testing on this for larger wallets if it would be smart to migrate to IndexedDB, which is an SQL-like storage in the browser and wouldn't require us to keep everything in-memory. We can of course also partition the data using normal storage too.

We must perform tests and verification on the actual need for this, and should be done before the milestone tagged.