Add AUTO_REFRESH setting

[rphlo]

Implement enhancement #37

[belugame]

@rphlo thanks a lot for this feature. I m happy to merge it in. 2 things.
a) we need unit tests that cover it e.g. assert that the token expiry timestamp is (not) extended when expected.
b) I would make the 1 second threshold bigger to avoid hitting the database more than it is useful for this usecase. 5 or even 10 seconds seems reasonable for me

[rphlo]

@belugame what about using freezegun in tests? This would add a dependency but would bring many benefit in my point of view for the tests...

[belugame]

@rphlo sure, I m familiar with freezegun. Feel free to add it to tox.ini

[belugame]

@rphlo Thank you for the work again, I extended your unit tests and tweaked things elsewhere unrelated to your changes. Glad to have your review my changes. #105