Add two flush method

[waters222]

1. Add flush table method.
2. Add flush chain method.

[squeed]

Two things:

1. Please add tests
2. It looks like this broke gofmt - please reformat.

[squeed]

but thanks for the submission!

[waters222]

I did gofmt on the code and add test case for FlushChain.

[squeed]

Is there any reason you can't use ClearChain?

[waters222]

1. ClearChain leave an empty chain or newly create chain in the end, which sometimes its not the use case for me.
2. FlushChain provide an alternative method to manipulate chain for certain case, but I think by using ClearChain can do the same with some modified code.
3. I think ClearChain is bit too high level which include two commands. I think simpler is good.