Allow configuration of sql.DB pool

[AlekSi]

This issue extracts non-controversial part of #280:

New options:

• exporter.max-open-conns: Maximum number of open connections to the database (https://golang.org/pkg/database/sql/#DB.SetMaxOpenConns). By default 1 - keeps original behavior.
• exporter.max-idle-conns: Maximum number of connections in the idle connection pool (https://golang.org/pkg/database/sql/#DB.SetMaxIdleConns). By default 1 - keeps original behavior.
• exporter.conn-max-lifetime: Maximum amount of time a connection may be reused (https://golang.org/pkg/database/sql/#DB.SetConnMaxLifetime). By default 60s - keeps original behavior.

Please confirm that you are still OK with this, so I can work on PR.

[SuperQ]

Yes, that seems fine.