Joint Trajectory Server: Replace time.sleep() with rate.sleep()

[hello-amal]

Currently, the Joint Trajectory Server (JTS) uses time.sleep() to control the loop rate. This blocks the thread that is currently executing the action e.g., that thread cannot be used for other work while sleeping. That might be okay because the sleep is just for 0.1 seconds, but it may be better to replace it with a rate.sleep() which handles control of the thread back to the executor, and uses the next available thread when the sleep is done. However, making this change could introduce other issues (like not having threads available when the sleep is done). Thus, this change should be coupled with thorough testing to ensure the JTS still executes as expected.

See this comment thread for more details.