Use file_client in CompositeFg

[innerlee]

This enables faster backend such as memcached

[codecov]

Codecov Report

Merging #212 (559d62c) into master (0abd0fe) will increase coverage by 0.00%.
The diff coverage is 90.90%.

@@           Coverage Diff           @@
##           master     #212   +/-   ##
=======================================
  Coverage   82.27%   82.28%           
=======================================
  Files         145      145           
  Lines        6691     6699    +8     
  Branches      997      998    +1     
=======================================
+ Hits         5505     5512    +7     
  Misses       1083     1083           
- Partials      103      104    +1     

Flag	Coverage Δ
unittests	82.28% <90.90%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
mmedit/datasets/pipelines/matting_aug.py	94.89% <90.90%> (-0.27%)	⬇️

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[tuochi]

Hi! I'd like to ask the problem about using memcached as backend . I install memcached by using its office document, but i can't find the files named 'server_list_cfg' or 'client_cfg'. What tool memcached client you used? Could you give me an example about "server_list_cfg","client_cfg" and "sys_path"in your spare time?Thanks a lot!!!!!!!!!!!!!!!!!!!!!!!!!!!