mstate: use Dead channels rather than Dying for watcher and presence

mstate/watcher/watcher.go

 // The watcher package provides an interface for observing changes
 // to arbitrary MongoDB documents that are maintained via the
 // mgo/txn transaction package.
 package watcher
 
 import (
         "fmt"
         "labix.org/v2/mgo"
         "labix.org/v2/mgo/bson"
         "launchpad.net/juju-core/log"
@@ skipping 79 matching lines @@
         }()
         return w
 }
 
 // Stop stops all the watcher activities.
 func (w *Watcher) Stop() error {
         w.tomb.Kill(nil)
         return w.tomb.Wait()
 }
 
-// Dying returns a channel that is closed when the watcher is stopping
-// due to an error or because Stop was called explicitly.
-func (w *Watcher) Dying() <-chan struct{} {
-       return w.tomb.Dying()
+// Dead returns a channel that is closed when the watcher has stopped.
+func (w *Watcher) Dead() <-chan struct{} {
+       return w.tomb.Dead()
 }
 
 // Err returns the error with which the watcher stopped.
 // It returns nil if the watcher stopped cleanly, tomb.ErrStillAlive
 // if the watcher is still running properly, or the respective error
 // if the watcher is terminating or has terminated with an error.
 func (w *Watcher) Err() error {
        return w.tomb.Err()
 }
 
@@ skipping 277 matching lines @@
                                         }
                                 }
                         }
                 }
         }
         if iter.Err() != nil {
                 return fmt.Errorf("watcher iteration error: %v", iter.Err())
         }
         return nil
 }