Refactor bitmap iteration to align with MySQL source code

Changes:
- Refactpred `UnsignedMap`
- Refactored `VisibilityMap`

Suggested by: #813 (comment)

replication/row_event.go

@@ -427,7 +427,7 @@ func (e *TableMapEvent) Dump(w io.Writer) {
 	fmt.Fprintf(w, "Primary key prefix: %v\n", e.PrimaryKeyPrefix)
 	fmt.Fprintf(w, "Enum/set default charset: %v\n", e.EnumSetDefaultCharset)
 	fmt.Fprintf(w, "Enum/set column charset: %v\n", e.EnumSetColumnCharset)
-	fmt.Fprintf(w, "Invisible Column bitmap: \n%s", hex.Dump(e.VisibilityBitmap))
+	fmt.Fprintf(w, "Visible Column bitmap: \n%s", hex.Dump(e.VisibilityBitmap))
 
 	unsignedMap := e.UnsignedMap()
 	fmt.Fprintf(w, "UnsignedMap: %#v\n", unsignedMap)
@@ -618,16 +618,21 @@ func (e *TableMapEvent) UnsignedMap() map[int]bool {
 	if len(e.SignednessBitmap) == 0 {
 		return nil
 	}
-	p := 0
 	ret := make(map[int]bool)
-	for i := 0; i < int(e.ColumnCount); i++ {
-		if !e.IsNumericColumn(i) {
-			continue
+	i := 0
+	for _, field := range e.SignednessBitmap {
+		for c := 0x80; c != 0; c >>= 1 {
+			if !e.IsNumericColumn(i) {
+				continue
+			}
+			ret[i] = field&byte(c) != 0
+			i++
+			if uint64(i) >= e.ColumnCount {
+				return ret
+			}
 		}
-		ret[i] = e.SignednessBitmap[p/8]&(1<<uint(7-p%8)) != 0
-		p++
 	}
-	return ret
+	return nil
 }
 
 // CollationMap returns a map: column index -> collation id.
@@ -747,13 +752,18 @@ func (e *TableMapEvent) VisibilityMap() map[int]bool {
 	if len(e.VisibilityBitmap) == 0 {
 		return nil
 	}
-	p := 0
 	ret := make(map[int]bool)
-	for i := 0; i < int(e.ColumnCount); i++ {
-		ret[i] = e.VisibilityBitmap[p/8]&(1<<uint(7-p%8)) != 0
-		p++
+	i := 0
+	for _, field := range e.VisibilityBitmap {
+		for c := 0x80; c != 0; c >>= 1 {
+			ret[i] = field&byte(c) != 0
+			i++
+			if uint64(i) >= e.ColumnCount {
+				return ret
+			}
+		}
 	}
-	return ret
+	return nil
 }
 
 // Below realType and IsXXXColumn are base from: