Update clock.Clock handling to prevent timeouts.

[reedobrien]

clock.Clock.Advance intermittently advances the time before clock.After
adds an alarm to be notified. This uses a new WaitAdvance method that
only advances time after something is in clock.waiting. It also uses
the zerotime appropriately rather than time now.

More information about this can be found on the wiki: cf.
https://github.com/juju/juju/wiki/Intermittent-failures
There was also discussion on IRC about the use of alarms; cf.
https://irclogs.ubuntu.com/2016/09/30/%23juju-dev.txt

Fixes: https://bugs.launchpad.net/juju/+bug/1607044

QA:

1. Run unit tests, all pass
2. Run the TestUpdatesAfterPeriod under stress an ensure the test no longer fails intermittently. (before fix it failed easily in <15 runs after not after 5000).

[babbageclunk]

Looks good!

[reedobrien]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju