Prevent drafts happening without a randomness seed

[izqui]

Right now, if the first draft of a term happens more than 256 blocks after the term starts, the blockhash that will be returned for the randomnessBN of the term will be 0.

Options to work around this problem:

• Treat this as a round being drafted in the wrong term and require rescheduling as in Reschedule adjudication rounds if the draft term was missed #14
• Implement a secondary heartbeat() function (which can be called one block after the actual heartbeat) that stores the randomness for the term. The caller could get the current dependingDrafts * heartbeatFee that is being paid to the heartbeat() caller and pay heartbeat fees just from activation, updates and deactivation fees (Charge jurors a fee for the activation and deactivation term heartbeats #7)

[bingen]

Some context: #6 (comment)

[bingen]

I think both options are complementary and that even if we implemented the second one we would still need the first one to make sure we have proper randomness.
Besides now we already are able to delay draft to later terms (#44), so I'm going to do the first one and we can still add the extra heartbeat function later.

[facuspagnuolo]

Closed by #58