Poll and cache account balances

[piyalbasu]

In order to prevent fetching /account-balances every single time we load the Account view, we can cache the value so we don't need to make this blocking call on every page load. To get data updates we can do the following:

• in the background of Account, poll /account-balances for updates
• opportunistically fetch /account-balances when we suspect a change has occurred (after completing send/swap, adding an asset)

Open questions:

• How should we animate in deltas to account balance updates
• How often should we poll? Do we need to be concerned with rate limiting or overloading the BE?