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.

该 issue 没有指明文件、测试或入口点。首先跟踪 issue 中描述的索引流程，并确定每个保存步骤中的失败会如何影响区块下载；当独立的数据保存失败不再阻止无关的索引或其 API 时，即视为完成。

由索引模型根据 Issue 内容生成。