* [Refactor] Refactor fileio but without breaking bc

* handle compatibility

* fix format

* modify io functions

* fix ut

* fix ut

* rename method names

* refine

* refine docstring

* fix ut in windows

* update ut

* minor fix

* ensure client is not None when closing it

* add more examples for list_dir_or_file interface

* refine docstring

* refine deprecated info

* fix ut

* add a description for lmdb docstring

[Refactor]: migrate LMDB backends and modifications from open-mmlab/mmcv#1764 and  open-mmlab/mmcv#1747 (#426)

* Add fileio ut

* pick modification from master branch

* pick modification from mmcv #1764

* test fileio without mmcv

* remove unnecessary checking