Consider renaming `DuplicateBlockVotePool`

[akhi3030]

I also dislike the name DuplicateBlockVotePool and "duplicate block" itself for that matter... Maybe we could rename it to BlockHashVotePool or to BlockVotePool (and SimpleVotePool to SlotVotePool)?

Originally posted by @qkniep in #387 (comment)

[wen-coding]

I'd personally prefer BlockHashVotePool, or VoteWithHashPool. We can see if @carllin has any preference.