Add mutex to progress bar for concurrency safe incrementing

Our progress bar is not concurrency safe since many go routines may be
attempting to modify the values at the same time. Add a mutex to be used
by the progress bar for safe incrementing in go routines.

Without this fix the following issue is found by the golang race
detector

```
WARNING: DATA RACE
Write at 0x00c0005a2b40 by goroutine 59:
  github.com/greenplum-db/gpbackup/utils.(*VerboseProgressBar).Increment()
      /home/kyeap/workspace/gpbackup/utils/progress_bar.go:77 +0xbc
  github.com/greenplum-db/gpbackup/restore.executeStatementsForConn()
      /home/kyeap/workspace/gpbackup/restore/parallel.go:59 +0xaa
  github.com/greenplum-db/gpbackup/restore.ExecuteStatements.func1()
      /home/kyeap/workspace/gpbackup/restore/parallel.go:164 +0x209
  github.com/greenplum-db/gpbackup/restore.ExecuteStatements.func2()
      /home/kyeap/workspace/gpbackup/restore/parallel.go:165 +0x41

Previous read at 0x00c0005a2b40 by goroutine 60:
  github.com/greenplum-db/gpbackup/utils.(*VerboseProgressBar).Increment()
      /home/kyeap/workspace/gpbackup/utils/progress_bar.go:81 +0xe5
  github.com/greenplum-db/gpbackup/restore.executeStatementsForConn()
      /home/kyeap/workspace/gpbackup/restore/parallel.go:59 +0xaa
  github.com/greenplum-db/gpbackup/restore.ExecuteStatements.func1()
      /home/kyeap/workspace/gpbackup/restore/parallel.go:164 +0x209
  github.com/greenplum-db/gpbackup/restore.ExecuteStatements.func2()
      /home/kyeap/workspace/gpbackup/restore/parallel.go:165 +0x41
```

utils/progress_bar.go

@@ -5,6 +5,7 @@ package utils
  */
 
 import (
+	"sync"
 	"time"
 
 	"github.com/greenplum-db/gp-common-go-libs/gplog"
@@ -61,6 +62,7 @@ type VerboseProgressBar struct {
 	prefix             string
 	nextPercentToPrint int
 	*pb.ProgressBar
+	mu sync.Mutex
 }
 
 func NewVerboseProgressBar(count int, prefix string) *VerboseProgressBar {
@@ -69,6 +71,8 @@ func NewVerboseProgressBar(count int, prefix string) *VerboseProgressBar {
 }
 
 func (vpb *VerboseProgressBar) Increment() int {
+	vpb.mu.Lock()
+	defer vpb.mu.Unlock()
 	vpb.ProgressBar.Increment()
 	if vpb.current < vpb.total {
 		vpb.current++