bug: 'database is locked' error after some time without calling the API

[lucasmenendez]

After a period of inactivity, the API returns an internal server error caused by a database error: error saving the census on the database error="database is locked".

After a few requests with this error, the API starts working without any issue.

At the moment we do not know how much time of inactivity is required to reproduce the error.

Original trace:

2023-08-29T10:20:00.095Z ERR api/censuses.go:188 > error saving the census on the database error="database is locked"
2023-08-29T10:20:10.521Z ERR api/censuses.go:188 > error saving the census on the database error="database is locked"
2023-08-29T10:20:17.18Z ERR api/censuses.go:188 > error saving the census on the database error="database is locked"
2023-08-29T10:23:17.608Z ERR api/censuses.go:188 > error saving the census on the database error="database is locked"

We found some references about SQLite database is locked error on the official documentation of the SQL driver that we use (mattn/go-sqlite3) and in its issues mattn/go-sqlite3#274 (comment).

[lucasmenendez]

The implemented fix comes from https://github.com/vocdoni/vocdoni-node/blob/0e0af7447473d98affff2e4ccd2217a45ec95846/vochain/indexer/indexer.go#L120-L143

[mvdan]

maybe squash the commits, since you ended up just copying what the indexer does