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

backend/compress/compress.go

@@ -38,6 +38,7 @@ import (
 const (
 	initialChunkSize = 262144  // Initial and max sizes of chunks when reading parts of the file. Currently
 	maxChunkSize     = 8388608 // at 256 KiB and 8 MiB.
+	chunkStreams     = 0       // Streams to use for reading
 
 	bufferSize          = 8388608
 	heuristicBytes      = 1048576
@@ -1362,7 +1363,7 @@ func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (rc io.Read
 		}
 	}
 	// Get a chunkedreader for the wrapped object
-	chunkedReader := chunkedreader.New(ctx, o.Object, initialChunkSize, maxChunkSize)
+	chunkedReader := chunkedreader.New(ctx, o.Object, initialChunkSize, maxChunkSize, chunkStreams)
 	// Get file handle
 	var file io.Reader
 	if offset != 0 {