Wallet scalability work

[mystenmark]

There are a number of poorly scaling elements of the current wallet design, e.g.

• Polling - active wallets continuously poll a fullnode to learn of updates.
• No incremental fetching - wallet repeatedly refetches all object data.

The recent change to prevent the browser wallet from polling while the tab is not focused put out the immediate fire, but we will eventually hit the same problems as the number of installed wallets grows.