Plugin storage improvements

[James-Mart]

There are various improvements that can be made to plugin storage.

As mentioned in the PR that delivered the MVP:

• Currently a known bug in the implementation of list-keys, so it is currently commented out and unavailable everywhere.
• The increment function is supposed to be atomic, but it is not. It currently just outputs a warning that it is non-atomic.

As well as fixing both of the above issues, the following are also important:

• Back pluginstorage with an IndexedDB API rather than LocalStorage. Currently plugin storage has the (massive) limitation that it can store a maximum of 5MB, whereas IndexedDB will give us essentially arbitrarily large storage capacity. The problem is that IndexedDB is exclusively an async interface, and therefore it would be easiest to wait until JSPI is supported in modern browsers to use it.
• Come up with a strategy for plugin storage writes to be dependent on the successful execution of the scheduled transaction. This way client storage remains consistent by following forks.