Make mempool cache a proper LRU (#2407)

Closes #2399

CHANGELOG_PENDING.md

@@ -21,6 +21,7 @@ FEATURES:
   * \#2310 Mempool is now aware of the MaxGas requirement
 
 IMPROVEMENTS:
+- [mempool] [\#2399](https://github.com/tendermint/tendermint/issues/2399) Make mempool cache a proper LRU (@bradyjoestar)
 - [types] add Address to GenesisValidator [\#1714](https://github.com/tendermint/tendermint/issues/1714)
 - [metrics] `consensus.block_interval_metrics` is now gauge, not histogram (you will be able to see spikes, if any)
 

mempool/mempool.go

@@ -577,7 +577,8 @@ func (cache *mapTxCache) Push(tx types.Tx) bool {
 
 	// Use the tx hash in the cache
 	txHash := sha256.Sum256(tx)
-	if _, exists := cache.map_[txHash]; exists {
+	if moved, exists := cache.map_[txHash]; exists {
+		cache.list.MoveToFront(moved)
 		return false
 	}