Remove counter from every mempoolTx (#2891)

Within every tx in the mempool, we store a 64 bit counter,
as an index for when it was inserted into the mempool.
This counter doesn't really serve any purpose.
It was likely added for debugging at one point,
Removing the counter reclaims memory,
which enables greater mempool sizes / mitigates resources at the same size.

Closes #2835

CHANGELOG_PENDING.md

@@ -27,5 +27,6 @@ program](https://hackerone.com/tendermint).
 
 - [config] \#2877 add blocktime_iota to the config.toml (@ackratos)
 - [mempool] \#2855 add txs from Update to cache
+- [mempool] \#2835 Remove local int64 counter from being stored in every tx
 
 ### BUG FIXES:

mempool/mempool.go

@@ -131,7 +131,6 @@ type Mempool struct {
 	proxyMtx             sync.Mutex
 	proxyAppConn         proxy.AppConnMempool
 	txs                  *clist.CList    // concurrent linked-list of good txs
-	counter              int64           // simple incrementing counter
 	height               int64           // the last block Update()'d to
 	rechecking           int32           // for re-checking filtered txs on Update()
 	recheckCursor        *clist.CElement // next expected response
@@ -167,7 +166,6 @@ func NewMempool(
 		config:        config,
 		proxyAppConn:  proxyAppConn,
 		txs:           clist.New(),
-		counter:       0,
 		height:        height,
 		rechecking:    0,
 		recheckCursor: nil,
@@ -365,9 +363,7 @@ func (mem *Mempool) resCbNormal(req *abci.Request, res *abci.Response) {
 			postCheckErr = mem.postCheck(tx, r.CheckTx)
 		}
 		if (r.CheckTx.Code == abci.CodeTypeOK) && postCheckErr == nil {
-			mem.counter++
 			memTx := &mempoolTx{
-				counter:   mem.counter,
 				height:    mem.height,
 				gasWanted: r.CheckTx.GasWanted,
 				tx:        tx,
@@ -378,7 +374,6 @@ func (mem *Mempool) resCbNormal(req *abci.Request, res *abci.Response) {
 				"res", r,
 				"height", memTx.height,
 				"total", mem.Size(),
-				"counter", memTx.counter,
 			)
 			mem.metrics.TxSizeBytes.Observe(float64(len(tx)))
 			mem.notifyTxsAvailable()
@@ -613,7 +608,6 @@ func (mem *Mempool) recheckTxs(txs []types.Tx) {
 
 // mempoolTx is a transaction that successfully ran
 type mempoolTx struct {
-	counter   int64    // a simple incrementing counter
 	height    int64    // height that this tx had been validated in
 	gasWanted int64    // amount of gas this tx states it will require
 	tx        types.Tx //