[Reviewer: Mike] Move memcached to use absolute times

Fixes https://github.com/Metaswitch/cpp-common/issues/11, but not yet live tested. I propose to fix by running the live tests (to make sure nothing expires too early and breaks things), then to test by:

* Sending a REGISTER and getting a 401
* Waiting for 35 seconds (so that the 40-second nonce doesn't expire from memcached)
* Sending the follow-up REGISTER
* Checking it succeeds because the nonce hasn't expired before the expected time

* Sending a REGISTER and getting a 401
* Waiting for 45 seconds (so that the 40-second nonce expires from memcached)
* Sending the follow-up REGISTER
* Checking it fails because the nonce has expired at the expected time