Add better support for handling new trajectories while the previous one is being executed

[AndrejOrsula]

Among others, here are some things to consider

• Cancelling request for a new trajectory does not make sense in most cases
• Queuing trajectory often does not make sense because end point of previous trajectory might not be the same as start point of the new one
• The current behaviour might be ideal if each trajectory consists of only a single point as it could allow smooth transition.
• If new trajectory does not affect joints from the current trajectory, then it should be fine to execute them simultaneously
  • E.g. joints and gripper can be totally independent

[AndrejOrsula]

It is difficult to select the best course of actions here. The current implementation might already be desirable.

For now, I leave it up to the user to make sure to wait until trajectory is executed with ign_moveit2.wait_until_executed()

[AndrejOrsula]

I believe the current behaviour is satisfactory. Reopen if needed