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.
Merge pull request #724 from mjs/no-SetEnvironAgentVersion-if-upgrading

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.