Always lock waddrmgr for every timeout or explicit request.

Use waddrmgr.IsError to avoid logging errors when trying to lock an
already locked wallet.

Fixes #349.

wallet/wallet.go

@@ -693,22 +693,19 @@ out:
 			break out
 
 		case <-w.lockRequests:
-			timeout = nil
 		case <-timeout:
 		}
 
 		// Select statement fell through by an explicit lock or the
 		// timer expiring.  Lock the manager here.
-		if timeout != nil {
 		timeout = nil
 		err := w.Manager.Lock()
-			if err != nil {
+		if err != nil && !waddrmgr.IsError(err, waddrmgr.ErrLocked) {
 			log.Errorf("Could not lock wallet: %v", err)
 		} else {
 			w.notifyLockStateChange(true)
 		}
 	}
-	}
 	w.wg.Done()
 }