Replaces sleep based waiting for log entry lookups

[sr-gi]

Replaces time.sleep in the watchtower-plugin tests to wait for certain things to happen on the backend by wait_for_logs, which should be less error-prone.

Also does some reformating and removes unnecessary imports.

Fixes #163 #156 #90

[sr-gi]

@booklearner you may be interested in this (b/c of #90)

[sr-gi]

I wanted to also address #156 here but I'm still unable to make it work consistently :(

[booklearner]

I was curious to see where wait_for_logs how implemented, adding here for reference: https://github.com/ElementsProject/lightning/blob/2ed10196d092d7aba22a700b0f26d248d04c36f7/contrib/pyln-testing/pyln/testing/utils.py#L303-L341

[sr-gi]

I've improved the regexs to look for to be a bit less broad in fddd477

[sr-gi]

I ended up managing to fix #156! 🎉