rename thermos_executor to aurora_executor

thermos_executor is no longer thermos-specific. there are some vestiges of thermos-specific task bits and states e.g. in executor_base that were missed in the open-source split, but they should be removed. i'd love to see this done before our first public release.

instead, thermos should be treated as a plugin to the aurora executor, much like all the health managers are now plugins.