Fix: db pool sharing

[Meldiron]

What does this PR do?

Improves connection sharing logic in pools using following vars:

• _APP_CONNECTIONS_MAX - Maximum amount of connections to a resource (in our case MySQL max connections)
• _APP_POOL_CLIENTS - Amount of containers to share max connections between
• _APP_SERVER_MULTIPROCESS - enabled/disabled. Must be enabled if process is using swoole async-style - spawning new process for every worker

Test Plan

No

Related PRs and Issues

x

Have you added your change to the Changelog?

No

Have you read the Contributing Guidelines on issues?

Yes