Add Heartbeat API

System heartbeat is a service for applications to synchronize their activity to save battery use time.

The main idea is that applications that must do periodic activity - after being in sleep a certain period - do that at the same time: for example send network "alive" messages at the same time (i.e. turn the wireless radio on at the same time).

The service is not only for network-aware applications: in fact it must be used by any applications that need to periodic wake-ups.

The other functions needed are wait (mintime, maxtime) and IWokeUp
Conceptually all processes are setting their preferred times and then a suitable scheduler sets a model where many processes will do batch access of network and CPU time. wait function should have the mintime, maxtime to allow scheduler understand roughly the needs.