Integrate networking with consensus protocol

[gavofyork]

[rphmeier]

We discussed this when the issue was created -- there is actually a fair bit of work to be done in terms of "integration", and only some of it is relevant to networking:

• input and output channels to the other validators
• stream of empty statement buffers (controlled by network so it can set the target validators)
• fetching candidate block-data and auxiliary data to check validity and availability

The rest of the work involves integration with the state:

• determination of roles taken by validators (in terms of grouping and primary selection)
• collation of local parachain candidate
• creation of a relay chain block from candidates with enough requisite votes
• verification that the candidates in a proposed block have enough requisite votes

As such, this might be worth splitting into two issues.

[gavofyork]

for substrate (poc-1) we don't need to be concerned with parachains.

for the relay chain block, we can probably just have a rotating validator propose it, no?

[gavofyork]

#54