Implement an api to callback a grain on a regular basis

there is need for reccurent events (eg server 'ticks') and a standrd way to 
implement them should be implemented


Caveats:
wide array of ways to implement this, some are more robust to network failures 
than others and this may need to be investigated

is this callback based? 'event stream', blocking call (sleep syscall) based?

there is need for 2 types of timers (hence timers/reminders) that do/do not 
activate the grain being called

how do we handle cleanup if a non activating timer goes off and the grain is 
not there (ie a game that has finihed or a player that has logged out), does it 
get registered? or cleaned up implicitly?

Ideas:
timers do not activate a grain while reminders will
implement as a decorator?
what is the backend? a remote grain? a timerfd? reusing the sleep mechanism?
