create task engine and run termination handler goroutine before warm pools polling

[lydiafilipe]

Summary

In this change, the termination handler goroutine is started before polling for instance target lifecycle state begins.
The windows process expects the termination handler to be invoked before it waits on the agent executing. Currently, since the termination handler is not being invoked when polling occurs, the process will not wait at all and exit after about a minute.
With this change, the goroutine for the termination handler will be started sooner.

Implementation details

The termination handler goroutine is moved out of startAsyncRoutines to start before the warm pools polling. Since the handler needs the task engine created, the task engine is now created before polling, however will still be initialized at a later point.

Testing

• make test on both linux and windows
• ran and stopped agent on both windows and linux, both with warm pools check enabled and not enabled. Checked that logs contained expected messaging.
  Linux (warm pools check enabled):

level=info time=2021-11-03T21:49:42Z msg="Agent received termination signal: terminated" module=termination_handler.go
level=debug time=2021-11-03T21:49:42Z msg="Shutting down task engine for final save" module=termination_handler.go
level=debug time=2021-11-03T21:49:42Z msg="Saving state before shutting down" module=termination_handler.go
level=info time=2021-11-03T21:49:42Z msg="Event stream ContainerChange stopped listening..." module=eventstream.go

Windows (warm pools check enabled):

level=info time=2021-11-03T21:45:33Z msg="Stopping Windows service" module=agent_windows.go
level=info time=2021-11-03T21:45:33Z msg="Received normal signal from Windows to exit" module=agent_windows.go
level=info time=2021-11-03T21:45:33Z msg="Termination handler received signal to stop" module=agent_windows.go
level=debug time=2021-11-03T21:45:33Z msg="Shutting down task engine for final save" module=termination_handler.go
level=debug time=2021-11-03T21:45:33Z msg="Saving state before shutting down" module=termination_handler.go
level=info time=2021-11-03T21:45:33Z msg="Bye Bye! Exiting with 0" module=agent_windows.go

New tests cover the changes: yes

Description for the changelog

Run termination handler goroutine before warm pools polling

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.