Allow assets to load in main screen before prices

[diegogurpegui]

When the apps loads the main screen, it tries to fetch the prices before listing all the assets.
From a UX perspective, it would be better to load the assets with the last price, while refreshing the latest prices in the background.

Tech note: In order to achieve this we need to store the latest price in the Asset main JSON data structure.

[diegogurpegui]

Already working but not great.
The assets load instantly, but with very outdated prices. We need to keep those up to date as much as possible so the "loading" mode doesn't display something too inaccurate.