Beacon chain block import

[mkalinin]

Addresses a backbone implementation for block processing part. It should have a modular design and connect all parts of import process together:

• block verification, depends on Beacon chain block production #1141, Validators shuffling #1142
• state transition Beacon chain state transition #1143
• fork choice rule, suggesting to use "first best" approach for the moment
• re-orgs
• block and state storage

"First best" algorithm is pretty simple, among N blocks with same slot number the best one is a block that has been imported first.

There is no necessity to implement this task apart from the others. It can be covered by work done within the other tickets.

[mkalinin]

A note to state storage step:
The spec decouples active state from crystallized state. At the first glance there is no need for that, hence I propose an implementation with a single state storage that holds both states described by the spec. Let's just try to check whether single state works or not.