* #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
if we call it after, we might receive a "fresh" transaction from
`broadcast_tx_sync` before old transactions (which were not
committed).
Refs #1091
```
Commit is called with a lock on the mempool, meaning no calls to CheckTx
can start. However, since CheckTx is called async in the mempool
connection, some CheckTx might have already "sailed", when the lock is
released in the mempool and Commit proceeds.
Then, that spurious CheckTx has not yet "begun" in the ABCI app (stuck
in transport?). Instead, ABCI app manages to start to process the
Commit. Next, the spurious, "sailed" CheckTx happens in the wrong place.
```