Remove repetitive interface lookups

[mhubii]

The repetitive interface lookups introduced with jazzy-2.3.0 for the ros2_control jazzy migration should be removed.

Use the new handles to avoid repetitive lookups instead (once repos synced):

• get_state_interface_handle
• get_command_interface_handle

Introduced in ros-controls/ros2_control#2831.

Note, this PR also indicates that the previous get_state / set_state / get_command / set_command, e.g. used in

lbr_fri_ros2_stack/lbr_ros2_control/src/system_interface.cpp

Line 210 in 9125bbc

set_state(state_keys_.commanded_joint_position[i], lbr_state_.commanded_joint_position[i]);

Are now non-realtime safe.

Related to ros-controls/ros2_control#2816.