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/repository/master_index.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"runtime"
 	"sync"
 
 	"github.com/restic/restic/internal/debug"
@@ -291,8 +292,6 @@ func (mi *MasterIndex) MergeFinalIndexes() error {
 	return nil
 }
 
-const saveIndexParallelism = 4
-
 // Save saves all known indexes to index files, leaving out any
 // packs whose ID is contained in packBlacklist from finalized indexes.
 // The new index contains the IDs of all known indexes in the "supersedes"
@@ -376,8 +375,10 @@ func (mi *MasterIndex) Save(ctx context.Context, repo restic.SaverUnpacked, pack
 		return nil
 	}
 
+	// encoding an index can take quite some time such that this can be both CPU- or IO-bound
+	workerCount := int(repo.Connections()) + runtime.GOMAXPROCS(0)
 	// run workers on ch
-	for i := 0; i < saveIndexParallelism; i++ {
+	for i := 0; i < workerCount; i++ {
 		wg.Go(worker)
 	}
 	err = wg.Wait()