cmd/jujud: synchronise state server upgrades

Use state's UpgradeInfo to synchronise state server upgrades. Overview:

- after restarting into a new tools version, master state servers will
  wait up to 15 minutes for any other state servers to indicate they
  are ready for upgrade. A downgrade to the previous tools version
  will occur if this wait times out.
- secondary state servers will wait until the master has completed its
  upgrade (to allow DB schema migrations to happen first)
- as state servers complete their upgrades they report this to the
  UpgradeInfo document. The document is archived once the last state
  server completes its upgrade.
Add new wrench points for upgrade-steps worker