fs/accounting: rework to enable accounting to work with crypt and b2

This removes the old system of part accounting and replaces it with a
system of popping off the accounting reader and wrapping up new ones
as necessary.

This makes it much easier to carry the context down the chain of
wrapped readers and get the limiting as near as possible to the
output.  This makes the accounting more accurate and the bandwidth
limiting smoother.

Fixes #2029 and Fixes #1443

backend/b2/upload.go

@@ -70,6 +70,7 @@ type largeUpload struct {
 	f        *Fs                             // parent Fs
 	o        *Object                         // object being uploaded
 	in       io.Reader                       // read the data from here
+	wrap     accounting.WrapFn               // account parts being transferred
 	id       string                          // ID of the file being uploaded
 	size     int64                           // total size
 	parts    int64                           // calculated number of parts, if known
@@ -126,10 +127,14 @@ func (f *Fs) newLargeUpload(o *Object, in io.Reader, src fs.ObjectInfo) (up *lar
 	if err != nil {
 		return nil, err
 	}
+	// unwrap the accounting from the input, we use wrap to put it
+	// back on after the buffering
+	in, wrap := accounting.UnWrap(in)
 	up = &largeUpload{
 		f:     f,
 		o:     o,
 		in:    in,
+		wrap:  wrap,
 		id:    response.ID,
 		size:  size,
 		parts: parts,
@@ -221,7 +226,7 @@ func (up *largeUpload) transferChunk(part int64, body []byte) error {
 		opts := rest.Opts{
 			Method:  "POST",
 			RootURL: upload.UploadURL,
-			Body:    accounting.AccountPart(up.o, in),
+			Body:    up.wrap(in),
 			ExtraHeaders: map[string]string{
 				"Authorization":    upload.AuthorizationToken,
 				"X-Bz-Part-Number": fmt.Sprintf("%d", part),
@@ -331,7 +336,6 @@ func (up *largeUpload) Stream(initialUploadBlock []byte) (err error) {
 	errs := make(chan error, 1)
 	hasMoreParts := true
 	var wg sync.WaitGroup
-	accounting.AccountByPart(up.o) // Cancel whole file accounting before reading
 
 	// Transfer initial chunk
 	up.size = int64(len(initialUploadBlock))
@@ -392,7 +396,6 @@ func (up *largeUpload) Upload() error {
 	errs := make(chan error, 1)
 	var wg sync.WaitGroup
 	var err error
-	accounting.AccountByPart(up.o) // Cancel whole file accounting before reading
 outer:
 	for part := int64(1); part <= up.parts; part++ {
 		// Check any errors