* core/finality-grandpa: Improve code comments
* core/finality-grandpa: Rename VoteOrPrecommit to PrevoteOrPrecommit
According to the Grandpa paper [1]:
> A vote is a block hash, together with some metadata such as round
number and the type of vote, such as prevote or precommit, all signed
with a voter’s private key.
To reduce confusion this patch makes the code consistent with the
research paper.
[1] https://github.com/w3f/consensus/blob/master/pdf/grandpa.pdf
* core/finality-grandpa: Add comment for NetworkStream concept
* core/finality-grandpa: Improve round_communication doc comment
* core/finality-grandpa: Rename PrevoteOrPrecommit to Vote
* core/finality-grandpa: Represent NetworkStream state machine as enum
* core/finality-grandpa: Improve KeepTopics comment