external_storage: Local.URI always return abs path

[YuJuncen]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Parse file://foo/bar to external storage, and format it back via URI call.

2. What did you expect to see? (Required)

It returns file://foo/bar.

3. What did you see instead (Required)

It returns file:///foo/bar.

4. What is your TiDB version? (Required)

Current master.