Improve the federated directory update process

[paride]

The federated directory update process updateFromPeers() is very weak:

• It has to be run periodically by a cronjob. This was done every 20 minutes or so when Signal (TextSecure) was federated with CyanogenMod's WhisperPush. Users expect things to happen way faster.
• While a new user still has to be propagates to the other federated servers it is possible for the user to register on another server, ending with a malfunctioning account.

We need to find a better way to do this. For example a server should "push" registrations/deletions to its peers.