Bug 1006331 - Cache contacts above the fold in localStorage, so we can render them instant on app startup

1. At the bottom of the page vs. loading as first script in head tag with defer attribute, the defered script takes 56 ms. longer to load.
2. Setting first chunk in localstorage almost has no penalty. Takes 3 ms. of blocking code on Tarako
3. Reading first chunk neither. Takes few ms.

That doesn't work. F.e. I have person A and person C, so I monitor their groups. Now new person B gets added and the observer doesn't fire.

When importing we have a separate UI right? Doesn't happen in the background. We can stop the observer when importing and resume when import is done.