Alternative fix to getControllersByName mutex-locking requirements

`ControllerManager::getControllerByName()` requires that the `controllers_lock_` is locked. 

There was a previous fix to this implemented [here](https://github.com/willowgarage/ros_control/pull/15#issuecomment-13596141) but instead of having a public and protected function, it could alternatively be solved by using a reentrant / recursive mutex instead of a unique mutex. This could keep the API a little cleaner, and also allow for more convenience functions like `getControllerByName()`.