Use SteadyTime instead of Time to avoid jumps back in time when using NTP clock

[alvarovillena]

As reported by @iportolesrb , there is a timing issue when we set adjust_ros_timestamp to true in PCs with NTP clock adjustment.
ROS time uses the system clock, which can be moved backwards by NTP or a similar time synchronization protocol. We are using ROS time to calculate the appropriate timestamp, depending on the sensor timestamp, the ROS timestamp, and the average difference of times for the last messages.
Whenever the system clock is moved backwards, the topic will be published with a previous timestamp. This should not happen and breaks some systems (such as our 3D SLAM system).

Some references:

• PR that adds SteadyTime into rospp_core: [rostime] add SteadyTime ros/roscpp_core#57
• Previous discussion about this issue: Wall time should use CLOCK_MONOTONIC instead of CLOCK_REALTIME which is vulnerable to leapseconds ros/roscpp_core#33

Current behavior:

[EdCheriee]

Have you ever found a workaround for this? We have a similar issue with the time jumps when using default NTP synchronisation on ubuntu.