cache: Synchronize downloading

This commit adds code to synchronize downloading files to the cache.
Before, requests that came in for files currently downloading would fail
because the file was not completed in the cache. Now, the code waits
until the download is completed.

Closes #1278

internal/cache/cache.go

@@ -155,10 +155,7 @@ func (c *Cache) IsNotExist(err error) bool {
 
 // Wrap returns a backend with a cache.
 func (c *Cache) Wrap(be restic.Backend) restic.Backend {
-	return &Backend{
-		Backend: be,
-		Cache:   c,
-	}
+	return newBackend(be, c)
 }
 
 // BaseDir returns the base directory.