Remove unused TestBlockVotingBlockMaker function
Readability improvments to comments for core/quorum/ and raft/
Readability improvments to raft/speculative_chain.go
Readability improvments to docs/
* Move raft checkpoint logging to logger package.
This allows us to use it everywhere and turn it on / off from command
flags.
* Return an error when the pending block is requested in Raft mode.