core/finality-grandpa: Minor refactorings (#3825)

* 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