Solves problem described in [[ https://linear.app/comm/issue/ENG-1682/yarn-scripts-arent-exiting | ENG-1682 ]].
When there are multiple `dbQuery` at one time also multiple `loadPool()` can run asynchronously, which lead to problem visible at this debug logs:
{F159472}
which means we call `mysqlPromise.createPool(options)` and assign it to `pool` when there is an already defined connection, as a result, we override the previous one which is not being closed and cause e.g. scripts not exiting.
Also, each spawned worker has a separate pool connection so this fix should work without additional synchronization.