Add flashblock timing configuration options

[avalonche]

📝 Summary

• Add --flashblocks.build-at-interval-end flag to wait for interval before building first flashblock instead of building immediately
• Add --flashblocks.send-offset-ms to adjust flashblock send timing (positive = send later, negative = send earlier)
• Refactor payload loop to wait for signal before building flashblocks
• Fix log message in tx_manager (funding -> registering)

💡 Motivation and Context

Make flashblocks timing closer to every 200 ms

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Sampling local timing

✅ I have completed the following steps:

• Run make lint
• Run make test
• Added tests (if applicable)

[SozinM]

Why is index 5 flashblock in the middle?

[akundaz]

Why is index 5 flashblock in the middle?

This is because the leeway time is applied to the last flashblock. Here it's set to 100ms so fb 5 comes 100ms earlier