Pool: reduce false sharing, minor cleanup

Pragma pack not necessary because we anyway have static_assert on the size
Fewer configs for autotuning
Pass initial config via SendConfig (also ensures workers are ready)
Remove separate ThreadPool::config_
Remove unused Type()
Move AdvanceWorkerEpoch to MainWakeAndBarrier - avoids extra load

PiperOrigin-RevId: 810382947

Author: jan-wassenberg