Update block authoring logic to be compatible with Aura/GRANDPA

[rphmeier]

We will need to split up the attestation-gathering logic from the Proposer logic.
The Proposer will still look for a minimum number of attested-to parachains, and we will need something for evaluating blocks and making sure they have enough parachain candidates for the time, etc.

Most of the evaluate logic would be placed in an aura::CustomVerifier that checks to see if the authorship cycle has been instantiated for the parent of a block (doing it if not) and then waiting until enough attestations come in before importing the block.

This might lead to some issues around clock synchrony if we don't evaluate blocks for longer than the block time in Aura, but needs further investigation.

Depends on paritytech/substrate#1124

[rphmeier]

(this is mostly done in #38)

[rphmeier]

Remaining tasks (after #42):

• Pass through InherentData when authoring blocks
• Instantiate consensus proposer and network correctly in Service (after Do not start internal authorship worker in service substrate#1158)
• Integrate GRANDPA authority set in runtime
• Start GRANDPA worker in Service for both block authors and GRANDPAs