Add a clock to the apiserver.

[howbazaar]

Pass this clock to the pingers, and rewrite pinger tests using testing clocks.

Addresses http://pad.lv/1455627 by using a testing clock and explicit time advancing rather than relying on the host clock.

QA

Ran the pinger test under load in a stress loop. Now appears to be good.

[howbazaar]

$$merge$$

[jujubot]

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