Asynchronous stream frequency diagnostics

[kubja]

• This pull request modifies frequency diagnostics to run independently from the frame callbacks. In the current implementation, frequency is updated with frame callback when nodes were subscribed. The asynchronous setup is more suitable for sensor health monitoring and enables to detect that there is a connection issue even when no node is subscribed. The argument was also made in issue Frequency diagnostics not published until getNumSubscribers > 0 #1417 .
• The diagnostics messages for each stream are published alongside the temperature diagnostics update. The loop update rate was lowered from 10s to 1s.
• The function was tested with Intel Realsense D435i.

[mikepurvis]

Looks awesome! Thanks for taking a stab at this; your change also looks a lot simpler than what I was picturing, so that's great.