* #1920 try to fix race condition on proposal height for published txs
- related to create_empty_blocks=false
- published height for accepted tx can be wrong (too low)
- use the actual mempool height + 1 for the proposal
- expose Height() on mempool
* #1920 add initial test for mempool.Height()
- not sure how to test the lock
- can the mutex reference be of type Locker?
-- this way, we can use a "mock" of the mutex to test triggering
* #1920 use the ConsensusState height in favor of mempool
- gets rid of indirections
- doesn't need any "+1" magic
* #1920 cosmetic
- if we use cs.Height, it's enough to evaluate right before propose
* #1920 cleanup TODO and non-needed code
* #1920 add changelog entry
* When create_empty_blocks=false, we don't enterPropose until we
* receive a transaction, but if we then receive a complete proposal,
* we should enterPrevote. A guard in addProposalBlockPart was checking if
* step==Propose before calling enterPrevote, but we need it to be step<=Propose,
* since we may not have seen a tx.
* This was discovered by disabling mempool broadcast, sending txs to
* peers one a time, and observing their consensus logs.
* updateToState exits early if the state isn't new, which happens after
* fast syncing. This results in not sending a NewRoundStep message. The mempool
* reactor depends on PeerState, which is updated by NewRoundStep
* messages. If the peer never sends a NewRoundStep, the mempool reactor
* will think they're behind, and never forward transactions. Note this
* only happens when `create_empty_blocks = false`, because otherwise
* peers will move through the consensus state and send a NewRoundStep
* for a new step soon anyways. Simple fix is just to send the
* NewRoundStep message during updateToState even if exit early
