Result file storage: remove URL encoding from file paths

We ran into a few "File name too long" IOErrors recently when trying to store results to the local filesystem.

In all cases, this was a side effect of URL-encoded non-ASCII paths (in Cyrillic), which made them 3 times as long.

Such encoding should be unnecessary when storing to modern filesystems (hopefully everything is UTF-8 compliant nowadays), so this patch URL decodes the paths before both the put & get operations.

Obviously this will cause the loss of existing objects with special characters in their paths.