Instrumentation: Refactor billing logic to improve auditability and log all major steps

[sameh-farouk]

Currently, a lot of billing logic is hidden inside a black box. We don't have enough logs to audit any abnormal behavior in contract billing. Also, Things like overflowed calculations when encountered are handled implicitly without any logs.

The billing code is crucial and must be properly instrumented.

Benefits:

• Easier identification and diagnosis of issues.
• Gaining visibility into the application's behavior in production environments.

Additional context

https://github.com/threefoldtech/home/issues/1560

[sameh-farouk]

Update: Almost done