swift backend: fix premature release of semaphore in Load() & document

concurrency issue in List().

refactor wrapReader from b2 -> semaphore so it can be used elsewhere.

As per discussion in PR #1399.

internal/backend/b2/b2.go

@@ -137,31 +137,6 @@ func (be *b2Backend) Location() string {
 	return be.cfg.Bucket
 }
 
-// wrapReader wraps an io.ReadCloser to run an additional function on Close.
-type wrapReader struct {
-	io.ReadCloser
-	eofSeen bool
-	f       func()
-}
-
-func (wr *wrapReader) Read(p []byte) (int, error) {
-	if wr.eofSeen {
-		return 0, io.EOF
-	}
-
-	n, err := wr.ReadCloser.Read(p)
-	if err == io.EOF {
-		wr.eofSeen = true
-	}
-	return n, err
-}
-
-func (wr *wrapReader) Close() error {
-	err := wr.ReadCloser.Close()
-	wr.f()
-	return err
-}
-
 // IsNotExist returns true if the error is caused by a non-existing file.
 func (be *b2Backend) IsNotExist(err error) bool {
 	return b2.IsNotExist(errors.Cause(err))
@@ -192,14 +167,7 @@ func (be *b2Backend) Load(ctx context.Context, h restic.Handle, length int, offs
 
 	if offset == 0 && length == 0 {
 		rd := obj.NewReader(ctx)
-		wrapper := &wrapReader{
-			ReadCloser: rd,
-			f: func() {
-				cancel()
-				be.sem.ReleaseToken()
-			},
-		}
-		return wrapper, nil
+		return be.sem.ReleaseTokenOnClose(rd, cancel), nil
 	}
 
 	// pass a negative length to NewRangeReader so that the remainder of the
@@ -209,14 +177,7 @@ func (be *b2Backend) Load(ctx context.Context, h restic.Handle, length int, offs
 	}
 
 	rd := obj.NewRangeReader(ctx, offset, int64(length))
-	wrapper := &wrapReader{
-		ReadCloser: rd,
-		f: func() {
-			cancel()
-			be.sem.ReleaseToken()
-		},
-	}
-	return wrapper, nil
+	return be.sem.ReleaseTokenOnClose(rd, cancel), nil
 }
 
 // Save stores data in the backend at the handle.