Add a last_upgraded field to state

This makes it possible to write external monitoring (cf #2)

History.txt

@@ -12,3 +12,4 @@
 * Allow einhorn to signal a given worker multiple times
 * Add 'signal' command to Einhornsh
 * Add exponential backoff to spinup if new processes are dying before being acked
+* Add last_upgraded field to State

lib/einhorn.rb

@@ -56,7 +56,8 @@ def self.default_state
         :socket_path => nil,
         :pidfile => nil,
         :lockfile => nil,
-        :consecutive_deaths_before_ack => 0
+        :consecutive_deaths_before_ack => 0,
+        :last_upgraded => nil
       }
     end
   end

lib/einhorn/command.rb

@@ -291,6 +291,7 @@ def self.upgrade_workers
       # not positive this is the right behavior, but it's not
       # obviously wrong.)
       Einhorn::State.consecutive_deaths_before_ack = 0
+      Einhorn::State.last_upgraded = Time.now
 
       Einhorn::State.version += 1
       replenish_immediately