Custom pool implementation with telemetry

Replace poolboy with custom pool implementation:

• better support idle_timeout (database_idle_time)
• telemetry events
• min_idle_workers - ensure a minimum count of workers are started and available - useful when startup time of a worker is costly - how many additional idle direct connections ready inside pool always
• should be parition-able
• workers should only be allowed to be checked out based on some criteria or state
• don't Supervisor.terminate_child in process loop, do async

Maybe fork: https://github.com/general-CbIC/poolex