Improve stability

The current indexing process is a serial process.

downloading block > saving block info > saving transaction info > saving UTXO info > saving address info > saving address's balance info > ... etc

The problem is, the whole system is broken if a single process goes wrong. For example, saving the balance info should not affect the UTXO data and its API, but, the next block will not be downloaded if the balance logic is broken.

The issue names no files, tests, or entry points. Start by tracing the indexing sequence described in the issue and identify how failures in each saving step affect block downloading; done means independent data-saving failures no longer stop unrelated indexing or its API.

Escrito por el modelo de indexación a partir del texto del issue.