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.
Actually, there's no way to do this with mgo/txn. AFAIK, transaction assertions can only check the document being manipulated and this check is for a document in another collection. Unfortunately, mgo transactions are not as useful as conventional relational DB transactions.

SetEnvironAgentVersion already has other racy validation checks (see checkCanUpgrade) so I don't feel too bad about adding another.