add MySQL backend implementant and support all methods

[lrita]

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All feature has been tested at local. Signed-off-by: lrita [email protected]

[GordonTheTurtle]

Please sign your commits following these rules: https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work The easiest way to do this is to amend the last commit:

$ git clone -b "backend-mysql" [email protected]:lrita/libkv.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353933104
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.