[BUG] Context is reused despite distinct requests, md5 hash is identical

Hello,

unfortunately, the MD5 checksum of a ROS message is of the message definition itself, not of the message contents. It's meant to be used for comparing the version of a message.

Hence, the OMPL planner will re-use a context even with distinct requests, and re-plan between the same (now wrong) start/end states. ~~There does not appear to be any built-in way to compare ROS messages, supposedly because comparing floats is not well-defined.~~

Correction: `operator==` is implemented for ROS messages. That does require us to hold on to the whole message though. Maybe we could use a shared_ptr to avoid duplication between user code and planner code?