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/walker/walker_test.go

@@ -76,6 +76,10 @@ func (t TreeMap) LoadTree(ctx context.Context, id restic.ID) (*restic.Tree, erro
 	return tree, nil
 }
 
+func (t TreeMap) Connections() uint {
+	return 2
+}
+
 // checkFunc returns a function suitable for walking the tree to check
 // something, and a function which will check the final result.
 type checkFunc func(t testing.TB) (walker WalkFunc, final func(testing.TB))