beacon: catch-up mode shouldn't be by default

[nikkolasg]

After a sucessful DKG, drand automatically starts the beacon generation mode. It first looks if there is a beacon already saved. In that case, it assumes (wrongly) that it should be in catch-up mode. Catch-up mode simply means it waits for another drand daemon to request a beacon, so it can requests a beacon for the same round, hence catching up on the round number.

1. This catch-up mode should be left as an option, argument to the method BeaconLoop()
2. This catch-up mode is really trivially attacked, so more thoughts need to put into that.