offers: improve offer flow logs

[orbitalturtle]

This PR improves debugging logs by:

• Logging when we receive an unverified invoice
• Logging the introduction node in two instances: 1) The introduction node we pick in our invoice request reply path. 2) The introduction node provided in an invoice we are about to pay.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 0.00%. Comparing base (6b2171f) to head (d844f75).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #132 +/- ## ====================================== Coverage 0.00% 0.00% ====================================== Files 1 1 Lines 96 96 ====================================== Misses 96 96 ```

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