Change keyframe updater to use a future

[zthorson]

Description

Switch keyframe updater from a detached thread to a future. This will help ensure we do not fall behind, but dropping updates that take too long with an appropriate warning.

Reason

See https://github.com/laxnpander/OpenREALM/issues/38

Method / Design

• Futures allow us to check if the previous async thread has completed before starting a new one.

Testing

Compiled and run on:

• Ubuntu 20.04 - Desktop
• Custom OpenREALM wrapper (non-ROS)

Other Notes