change use of errors.Wrap to fmt.Errorf with %w verb

Closes #4603

Commands used (VIM):

```
:args `rg -l errors.Wrap`
:argdo normal @q | update
```

where q is a macros rewriting the `errors.Wrap` to `fmt.Errorf`.

mempool/errors.go

@@ -1,9 +1,8 @@
 package mempool
 
 import (
+	"errors"
 	"fmt"
-
-	"github.com/pkg/errors"
 )
 
 var (

mempool/reactor_test.go

@@ -1,6 +1,7 @@
 package mempool
 
 import (
+	"errors"
 	"net"
 	"sync"
 	"testing"
@@ -8,7 +9,6 @@ import (
 
 	"github.com/fortytw2/leaktest"
 	"github.com/go-kit/kit/log/term"
-	"github.com/pkg/errors"
 	"github.com/stretchr/testify/assert"
 
 	"github.com/tendermint/tendermint/abci/example/kvstore"