* 20456: add hot connection pool via the minConnections setting
* added the akka.http.host-connection-pool.min-connections setting to reference.conf
* modified config implementation s o it's being read, with 0 as default
* based on the setting two types of pools will be created - hot and cold
* hot ones will start ASAP and not close when the number of active connections fall to 0
* PR remarks: english, remove getIntOpt ...
Moving logic into PoolConductor WIP
* WIP: moving minConnections logic into PoolConductor, needs more precise tests
* added testing via expectNextN in ConnectionPoolSpec.scala; fixed the bug with skipping the actor state in PoolSlot
* fix comments from PR: make sure PoolConductor produces only SwitchSlotCommand
* fix typo: slotSettings -> slots
* Fix style after code review
- inline short snippets
- shorter Mima
- better naming
- make SlotCommands final
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