Executing term change block took too much time

In the Corgi network, executing term change block took too much time.

I copied the log from a Corgi node. Blocks Verifier #1's log message. "Blocks Verifier #1" thread does not print any log between two add_balance log. The gap is more than a second.
```
#164574181 2020-08-26 14:00:03.561690513 UTC Blocks Verifier #1 TRACE state add_balance(0x263b…fcb1, 0): 1029374756
#164574181 2020-08-26 14:00:05.957492963 UTC Blocks Verifier #1 TRACE state add_balance(0x6850…65e1, 2160): 1050470102
```

We can find this situation very frequently in Corgi. About 10 times in a day. It appears only in the term closing blocks.

The reason is that CodeChain reads all header in DB generated in a term. It is about 1000 headers.

從用於結束 term 的區塊之 Blocks Verifier 路徑開始，追蹤 CodeChain 如何從資料庫讀取某個 term 的所有 header。重現或測量 Corgi 中兩筆 add_balance 日誌項目之間的間隔，然後驗證 term change block 的執行不再產生報告中所述的數秒延遲。

由索引模型根據 Issue 內容生成。