offers: various small fixes

[orbitalturtle]

This PR fixes a few things:

• When choosing an introduction node for a reply path for an invoice request, we now exclude the node we're making a payment to as an introduction node option. I'm still thinking about whether this is the best approach, or whether it fully makes sense, but at least (for now) it seems to fix a problem when sending payments to CLN. If we send CLN an invoice request where the introduction node of the reply path is the CLN node, CLN is unsure how to respond and, well, doesn't respond.
• Improves logs when we've received an invoice or when attempting to verify an invoice results in an error.
• Lowers the timeout period for waiting for an invoice to come back. Though we should probably make this configurable at some point...

[codecov[bot]]

Codecov Report

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

Project coverage is 0.00%. Comparing base (27df32b) to head (dee0d89). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #115 +/- ## ====================================== Coverage 0.00% 0.00% ====================================== Files 1 1 Lines 77 77 ====================================== Misses 77 77 ```

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