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

vfs/vfscommon/options.go

@@ -79,6 +79,11 @@ var OptionsInfo = fs.Options{{
 	Default: fs.SizeSuffix(-1),
 	Help:    "If greater than --vfs-read-chunk-size, double the chunk size after each chunk read, until the limit is reached ('off' is unlimited)",
 	Groups:  "VFS",
+}, {
+	Name:    "vfs_read_chunk_streams",
+	Default: 0,
+	Help:    "The number of parallel streams to read at once",
+	Groups:  "VFS",
 }, {
 	Name:    "dir_perms",
 	Default: FileMode(0777),
@@ -171,6 +176,7 @@ type Options struct {
 	FilePerms          FileMode      `config:"file_perms"`
 	ChunkSize          fs.SizeSuffix `config:"vfs_read_chunk_size"`       // if > 0 read files in chunks
 	ChunkSizeLimit     fs.SizeSuffix `config:"vfs_read_chunk_size_limit"` // if > ChunkSize double the chunk size after each chunk until reached
+	ChunkStreams       int           `config:"vfs_read_chunk_streams"`    // Number of download streams to use
 	CacheMode          CacheMode     `config:"vfs_cache_mode"`
 	CacheMaxAge        fs.Duration   `config:"vfs_cache_max_age"`
 	CacheMaxSize       fs.SizeSuffix `config:"vfs_cache_max_size"`