s3: Use low level API for saving files

benchmark                        old ns/op       new ns/op       delta
    BenchmarkBackendMinio/Save-4     184482294       40663344        -77.96%
    BenchmarkBackendS3/Save-4        35030825568     54475455819     +55.51%

    benchmark                        old MB/s     new MB/s     speedup
    BenchmarkBackendMinio/Save-4     90.95        412.64       4.54x
    BenchmarkBackendS3/Save-4        0.48         0.31         0.65x

    benchmark                        old allocs     new allocs     delta
    BenchmarkBackendMinio/Save-4     631            560            -11.25%
    BenchmarkBackendS3/Save-4        646            584            -9.60%

    benchmark                        old bytes     new bytes     delta
    BenchmarkBackendMinio/Save-4     66818060      50735         -99.92%
    BenchmarkBackendS3/Save-4        66834000      73024         -99.89%

src/restic/backend/s3/s3.go

@@ -80,6 +80,10 @@ func (be *s3) Location() string {
 	return be.bucketname
 }
 
+type Sizer interface {
+	Size() int64
+}
+
 // Save stores data in the backend at the handle.
 func (be *s3) Save(h restic.Handle, rd io.Reader) (err error) {
 	if err := h.Valid(); err != nil {
@@ -88,6 +92,13 @@ func (be *s3) Save(h restic.Handle, rd io.Reader) (err error) {
 
 	objName := be.Filename(h)
 
+	var size int64
+	if r, ok := rd.(Sizer); ok {
+		size = r.Size()
+	} else {
+		panic("Save() got passed a reader without a method to determine the data size")
+	}
+
 	debug.Log("Save %v at %v", h, objName)
 
 	// Check key does not already exist
@@ -98,14 +109,14 @@ func (be *s3) Save(h restic.Handle, rd io.Reader) (err error) {
 	}
 
 	<-be.connChan
-	defer func() {
-		be.connChan <- struct{}{}
-	}()
 
-	debug.Log("PutObject(%v, %v)",
-		be.bucketname, objName)
-	n, err := be.client.PutObject(be.bucketname, objName, rd, "binary/octet-stream")
-	debug.Log("%v -> %v bytes, err %#v", objName, n, err)
+	debug.Log("PutObject(%v, %v)", be.bucketname, objName)
+	coreClient := minio.Core{be.client}
+	info, err := coreClient.PutObject(be.bucketname, objName, size, rd, nil, nil, nil)
+
+	// return token
+	be.connChan <- struct{}{}
+	debug.Log("%v -> %v bytes, err %#v", objName, info.Size, err)
 
 	return errors.Wrap(err, "client.PutObject")
 }