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.

Can we observe the groups that are above the fold? (Or the list if empty)

The resason for that is that we will be calling the callback each time that we add a new contact. In the case of importing contacts (fb, gmail, hotmail, sim, sdcard), we will be introducing several (as much as contacts are being imported) sync calls to localstorage, leading that to making slower the process of importing.