Improve log subscription handling

[castodius]

Currently the log subscription lambda runs every X minutes and aggressively tries to put a log subscription. I think we can be a bit smarter:

• Trigger the function whenever a new Log Group gets created and check if it matches the regular expressions
• Trigger the function whenever the SSM Parameter holding the regular expressions gets updated. This would rerun the logic on all groups
• Add some wait time in between the put attempts, it is far too easy to hit a 409 at the moment

If a user wants to reapply the subscriptions they would simply have to rerun the second function.

[castodius]

This would require removing the current caching that is taking place.

[castodius]

Partially improved in https://github.com/enfogroup/sniffles-cdk/commit/340f1b6258b0946a7d48a6c5fbdf737f3640de92, still a blunt solution.