state: infrastructure for synchronising state server upgrades

Introduce a new UpgradeInfo struct (and underlying mongo document) which will be used by state servers to synchronise tools upgrades.

Later commits will integrate this infrastructure with the upgrade machinery in jujud.

William Reade and Menno Smits both contributed to this revision.
I think this is too complicated

just inline the code of validate here as the test is really

did someone type SetStatus("upgrading, I think")