archiver: Limit blob saver count to GOMAXPROCS

Now with the asynchronous uploaders there's no more benefit from using
more blob savers than we have CPUs. Thus use just one blob saver for
each CPU we are allowed to use.

internal/archiver/archiver.go

@@ -122,7 +122,9 @@ func (o Options) ApplyDefaults() Options {
 	}
 
 	if o.SaveBlobConcurrency == 0 {
-		o.SaveBlobConcurrency = uint(runtime.NumCPU())
+		// blob saving is CPU bound due to hash checking and encryption
+		// the actual upload is handled by the repository itself
+		o.SaveBlobConcurrency = uint(runtime.GOMAXPROCS(0))
 	}
 
 	if o.SaveTreeConcurrency == 0 {