Implement Fork Choice Rule for Beacon Chain

[terencechain]

To resolve this issue, we will implement beacon chain fork choice rule for ethereum 2.0. We need to implement beacon chain as standalone PoS chain which tags alone with the PoW main chain. There is a dependent fork choice rule where the head of the beacon chain is the highest scoring beacon chain block along with a main chain block reference that points to the highest scoring main chain block. This implies that every beacon chain block will contain main chain block reference that has the highest score. Then the head of of the beacon chain block is the one with the highest beacon chain score.

Whenever beacon chain node receives a new block, it should apply the fork choice rule to keep track of the head of main and beacon chains. When the node receives a new block, it changes the head only if the block score is an improvement over the existing head and also the new block's main chain reference is in the main chain.

[rauljordan]

To update this, we have implemented all of the block processing + head update logic with a naive fork choice rule as a stub (see #422) that selects the first received processed block during a slot as canonical. A PR that successfully fulfills this issue will implement a minimal, immediate message driven GHOST protocol as a real fork choice rule in our beacon chain instead of our current stub. This will require a design doc before diving into implementation details, as it is a non-trivial feature to implement.