adapt workers based on whether an operation is CPU or IO-bound

Use runtime.GOMAXPROCS(0) as worker count for CPU-bound tasks,
repo.Connections() for IO-bound task and a combination if a task can be
both. Streaming packs is treated as IO-bound as adding more worker
cannot provide a speedup.

Typical IO-bound tasks are download / uploading / deleting files.
Decoding / Encoding / Verifying are usually CPU-bound. Several tasks are
a combination of both, e.g. for combined download and decode functions.
In the latter case add both limits together. As the backends have their
own concurrency limits restic still won't download more than
repo.Connections() files in parallel, but the additional workers can
decode already downloaded data in parallel.

internal/restorer/filerestorer.go

@@ -20,8 +20,6 @@ import (
 //      con: each worker needs to keep one pack in memory
 
 const (
-	workerCount = 8
-
 	largeFileBlobCount = 25
 )
 
@@ -51,6 +49,7 @@ type fileRestorer struct {
 	idx        func(restic.BlobHandle) []restic.PackedBlob
 	packLoader repository.BackendLoadFn
 
+	workerCount int
 	filesWriter *filesWriter
 
 	dst   string
@@ -61,13 +60,18 @@ type fileRestorer struct {
 func newFileRestorer(dst string,
 	packLoader repository.BackendLoadFn,
 	key *crypto.Key,
-	idx func(restic.BlobHandle) []restic.PackedBlob) *fileRestorer {
+	idx func(restic.BlobHandle) []restic.PackedBlob,
+	connections uint) *fileRestorer {
+
+	// as packs are streamed the concurrency is limited by IO
+	workerCount := int(connections)
 
 	return &fileRestorer{
 		key:         key,
 		idx:         idx,
 		packLoader:  packLoader,
 		filesWriter: newFilesWriter(workerCount),
+		workerCount: workerCount,
 		dst:         dst,
 		Error:       restorerAbortOnAllErrors,
 	}
@@ -150,7 +154,7 @@ func (r *fileRestorer) restoreFiles(ctx context.Context) error {
 		}
 		return nil
 	}
-	for i := 0; i < workerCount; i++ {
+	for i := 0; i < r.workerCount; i++ {
 		wg.Go(worker)
 	}