Expire the token prematurely on client side

Created by: klippx

Our assumption was that expires_at? is inaccurate and needs to be offset by some fixed latency amount that we never expect to exceed has been tested and this change solves the described issue #393 (closed)

The Kred::HttpErrors (which are due to getting two 401 in a row and failing the request) have vanished competely after the deploy (second green marker from the right).

The 10 seconds are a bit arbitrary, and in worst case (where latency is zero) will simply expire the token 10seconds too early on the client side. Which I think most people can live with since expires_at often is 300s or even 1800s.