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.

Comece pelo caminho do Blocks Verifier para os blocos de encerramento de term e rastreie como o CodeChain lê todos os headers do banco de dados para um term. Reproduza ou meça a diferença entre as duas entradas de log add_balance no Corgi e, em seguida, verifique se a execução de um term change block não incorre mais no atraso de vários segundos relatado.

Escrita pelo modelo de indexação a partir do texto da issue.