Add Engine::on_prepare_block, call randomness and validator set.

[afck]

This adds an Engine::on_prepare_block method that is called from within the miner right after a new block for mining has been opened. The transactions it returns are put directly into the block, without going through the queue.

This change also moves the calls to the randomness and validator set contracts into on_prepare_block, but the latter still puts the transactions in the queue for now.

See #75.

[afck]

Closing this for now; as discussed, let's keep putting transactions into the queue: that at least is more robust in cases where e.g. one validator is skipped multiple times.

[varasev]

Closing this for now; as discussed, let's keep putting transactions into the queue: that at least is more robust in cases where e.g. one validator is skipped multiple times.

@afck I forgot to mention that afck-new-block solved the issue I wrote in Slack about (I mean Block import failed for #2). The afck-new-block branch solved this but if we don't merge this into aura-pos, the error will still occur.

[afck]

Right! Now I'm confused why it solved that, though. 🤔
But at least we know that it was the first commit that did, and not the second…

[varasev]

Yes, we need the changes from this commit: 7e9a265

[afck]

I can't reopen (probably because I changed the branch), so I created #89.