updated randomness sampling in fil to broaden ec solution to all rand…

[sternhenri]

…omness given vdf removal

@whyrusleeping, this PR should do the following:

• remove stale references to multiple tickets
• generalize EC solution to null blocks to all ticket generation, namely: if attempting to draw a ticket from a null block, go back to prior parent and generate ticket with H(prior_ticket || epoch_num)
• redraw ticket chain abstraction and code accordingly
  • including changing ticket output to be a SHA256 output in all cases (either of the ticket gone through vrf -- for live blocks -- or of the ticket || round, when drawing from null).

In order to ease all of this, I moved most of the ticket things to SPC readme.

Needs to be made to compile still.

[sternhenri]

I only now get to your comments from #538 @jbenet. They were super useful, I'm sorry I missed them until now. And likewise, I now finally understand what you meant in your question there @whyrusleeping that I thought I had answered (was thinking solely in terms of EC randomness, not overall randomness). Sorry about that!

[zixuanzh]

Merging for velocity, @jzimmerman please review when you get a chance.

[jzimmerman]

Some general notes:

• I think it makes sense to have randomness primitives both in the runtime (as a VM context primitive available to actors) and in the relevant subsystems (as a pure function to extract randomness from chain state), but we shouldn't duplicate implementations (i.e., the former should be implemented in terms of calls to the latter).
• See VM updates #637 for serialization APIs -- ideally, we would do something like the following for random seeds, to avoid potentially error-prone duplication/variation in seed generation methods:

type RandomnessRole int
const (
  RandomnessRole_TicketProduction RandomnessRole = 1+iota
  RandomnessRole_ElectionProof
  // ...
)

type RandomSeed []byte
DeriveRandomSeedExtended(role RandomnessRole, s Serialization) RandomSeed
DeriveRandomSeed(role RandomnessRole) RandomSeed
// Serialization methods "Serialize_T(T) Serialization" auto-generated as in https://github.com/filecoin-project/specs/pull/637

Randomness(epoch Epoch, seed RandomSeed)

• We should set lookback parameters keeping in mind the possibility of null epochs.