Separate triggering of "load", "configure", and "start" of controllers

[destogl]

Using LifecycleNode's states there are three stages of starting a controller:

• Load: loads controller's plugin
• Configure: read parameters and initialize publishers/subscribers
• Start: Start controller

In ROS1, load and configure functionalities were executed in one step. So, if one wanted to start a controller with custom parameters on run-time, they were set before loading it on the parameter-server.
ROS2 does not allow to set parameters in a namespace of a non-existing node, therefore load and configure steps have to be separated. So, if one wants to set custom parameters on run-time (without using launch files) they have to, load the controller, set the parameter, and then configure it.

Bundling configure and start sages is not suitable for cases where on-the-fl" controllers-switching is used. First, this would slow down the starting process, and second, a controller configuration step could fail leaving a robot "uncontrolled".

For convenience, bundling services are provided:

• "load_and_configure"
• "load_and_start": executing all three steps

PRs #234 and #246 try to address this issue by bundling the stages. For details see discussions in them.
The issue "replaces" #168