Use writer in tls.Config to get the tls session

[xieyuschen]

When using a third part library, everything you want to do should check the followings:

1. Does the library provide such feature?
2. Does any library provide such feature?
3. Is it easy to modify the source library to achieve the feature you want?

If all of answers above are NO, then you can do yourself to provide a new feature. Instead, use another library to do your feature!