Add created_at attribute to each token

[nicholas-nicolau]

Hi, everyone.

Recently I faced a really strange problem and it took me a while to understand it fully.
I reduced the token expiration time from 3 weeks to some minutes and then the users couldn't log in my application anymore, I realized that the user was logging in, receiving a new token but the new token was not being persisted on the database.

The problem was that I allowed just one connected device (through 'max_number_of_devices' config) and the criteria to clear old tokens was based on the expiration time - leaving only the token that will take longer to expiry - so my new 'expiry in minutes token' had been deleted every time it was created.

I'm not sure about why there isn't a 'created_at' attribute to track tokens so I'll leave this issue here, if it seems promising I'll open a PR.