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/crypt/cipher.go

@@ -14,6 +14,7 @@ import (
 	"unicode/utf8"
 
 	"github.com/ncw/rclone/backend/crypt/pkcs7"
+	"github.com/ncw/rclone/fs/accounting"
 	"github.com/pkg/errors"
 
 	"golang.org/x/crypto/nacl/secretbox"
@@ -691,11 +692,12 @@ func (fh *encrypter) finish(err error) (int, error) {
 
 // Encrypt data encrypts the data stream
 func (c *cipher) EncryptData(in io.Reader) (io.Reader, error) {
+	in, wrap := accounting.UnWrap(in) // unwrap the accounting off the Reader
 	out, err := c.newEncrypter(in, nil)
 	if err != nil {
 		return nil, err
 	}
-	return out, nil
+	return wrap(out), nil // and wrap the accounting back on
 }
 
 // decrypter decrypts an io.ReaderCloser on the fly