This is an attempt to fix the intermittently failing
`TestPrepareProposalReceivesVoteExtensions` test in the internal
consensus package.
Occasionally we get prevote messages via the subscription channel, and
we're not interested in those. This change allows us to specify what
types of votes we're interested in (i.e. precommits) and discard the
rest.
Signed-off-by: Thane Thomson <connect@thanethomson.com>