Copy some maintenance preferences into bootstrap preferences

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Allow to store public key for unknown users (fixes #144)

Unknown users are now stored with "(unknown)" temporarly. They last
until next sync and the have the phone number equals to their hash.

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>