Refactor the "terminate agent" AWS lambda

[kayman-mk]

Describe the solution you'd like

During implementation of #628 some questions arose regarding this lambda function:

• can we update to Python 3.9?
• can we move to ARM to save costs?
• the best setting for the memory size?
• add a logging library (JSON output format)

Analyze these points and create a PR for it.

[kayman-mk]

Memory vs runtime:

128 MB --> < 20s

Startup time less than 1 second.

Checked several executions in my live account.

[kayman-mk]

python-json-logger seems to be a good library for logging (on top of logging)