state: don't allow SetEnvironAgentVersion if state server upgrades are in progress

An upgrade isn't going to be able to complete if an upgradeInfo document for another upgrade is still active.
Pity.  I'd hate for us to add yet another potential race-induced point of failure to our tests.

Is there anything else we can do to mitigate the cross-collection issue?  Perhaps the stored info could grow a field or two that would allow the assertion?  This would likely duplicate info, but for the sake of atomicity.  Or perhaps a mutex would be in order here?  Would either be worth it?  I don't know. :)