Fetch UTXOs without depending on a third party

[mijoco-btc]

The API currently uses mempool space to find UTXO sets for a given BTC address.

The application should not depend on external APIs. This can be achieved using a hosted utxo indexer. Potential options need to be evaluated;

• ElectrumX
• Hiro Mempool Service
• Bitcoin Sync

[igorsyl]

Hi @radiclear:

• Please assign issues to yourself (GH doesn't do this automatically)
• Also, making sure we can have a working system for Alpha.
• Please mark the issue in the project board with Uber-Milestone as Alpha or a future milestone.
• IMO, this issue is not critical for Alpha, so I'll tag it as an April milestone.