"Tie breaker rule": implement a method to decide between chains with the same weight

[irenegia]

Background
Two (or more) forks may have the same exact weight and in this case we need a rule to decide which one to extend.

Proposal
Implement the following rule already described in the spec link:

When selecting between Tipsets of equal weight, a miner chooses the one with the smallest final ticket.

In the case where two Tipsets of equal weight have the same min ticket, the miner will compare the next smallest ticket (and select the Tipset with the next smaller ticket). This continues until one Tipset is selected.

Note: we should use the ticket used as election proof (VRF(drand)) for this (updated spec).

CC @sa8 @Kubuxu

[nicola]

I propose that @sa8 writes this as an FIP in the FIP repo (https://github.com/filecoin-project/fips)

[sa8]

Link to FIP: https://docs.google.com/document/d/1VHm_0rh9XCcjJsrnpbLtbFpLNRmOgkEoNTNlt0NHiOQ/edit?usp=sharing

[nicola]

This was written up here: filecoin-project/FIPs#22

[jennijuju]

FIP: https://github.com/filecoin-project/FIPs/pull/156/files