Solves problem described in 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:
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.