Check and fix H2NVCache.shutdown

[acaproni]

In coderabbit review of #205, coderabbit reported the following message that needs to be investigated and, if it is the case, fixed:

Improve connection management

The database connection is created in the init() method, but there's no explicit connection closing in the shutdown() method. Consider adding connection closing to prevent resource leaks:

override def shutdown(): Unit = {
  if (conn != null && !conn.isClosed) {
    conn.close()
  }
  DeleteDbFiles.execute(connectionData.folderName, connectionData.fileName, false)
}

Also, consider using a connection pool for better performance and resource management, especially if this cache might be used in a multi-threaded environment.

Also applies to: 249-251