[server] avoid creating multiple connection pools

Summary:
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.

Test Plan: Run script (e.g. rename-user.js since it begins with multiple asynchronous queries) and check if it will exit adequately.

Reviewers: tomek, atul, ashoat

Reviewed By: tomek

Subscribers: ashoat, atul, abosh

Differential Revision: https://phab.comm.dev/D5050