Add a ParseURI function and remove the other URI parsing functions.

ParseURI provides all the functionality used by other filesystems right now, so we're able to remove their custom URI parsing code.

- Use ParseURI for URI parsing in GCS and HDFS.
- Note that this fixes a bug where FileSystem::TranslateName("file://foo/bar")
  gives "foo/bar" as the path. "file:///" is the correct prefix in that case.
Change: 134009580