chunkedreader: add --vfs-read-chunk-streams to parallel read chunks

This converts the ChunkedReader into an interface and provides two
implementations one sequential and one parallel.

This can be used to improve the performance of the VFS on high
bandwidth or high latency links.

Fixes #4760

fs/chunkedreader/sequential_test.go

@@ -0,0 +1,20 @@
+package chunkedreader
+
+import (
+	"testing"
+
+	_ "github.com/rclone/rclone/backend/local"
+	"github.com/rclone/rclone/fstest/mockobject"
+)
+
+func TestSequential(t *testing.T) {
+	content := makeContent(t, 1024)
+
+	for _, mode := range mockobject.SeekModes {
+		t.Run(mode.String(), testRead(content, mode, 0))
+	}
+}
+
+func TestSequentialErrorAfterClose(t *testing.T) {
+	testErrorAfterClose(t, 0)
+}