Updated default params and added write-and-flush option for persisted device clients

[ShvaykaD]

Pull Request description

• Updated default params in .yml file for persisted device clients after local performance tests.
• Added two new parameters:
  • MQTT_PERSISTENT_MSG_WRITE_AND_FLUSH
  • MQTT_PERSISTENT_BUFFERED_MSG_COUNT

mqtt:
  ...
  persistent-session:
    device:
      persisted-messages:
        ...
        # If enabled, each message is published to persistent DEVICE client subscribers with flush. When disabled, the messages are buffered in the channel and are flushed once in a while
        write-and-flush: "${MQTT_PERSISTENT_MSG_WRITE_AND_FLUSH:true}"
        # Number of messages buffered in the channel before the flush is made. Used when `MQTT_PERSISTENT_MSG_WRITE_AND_FLUSH` = false
        buffered-msg-count: "${MQTT_PERSISTENT_BUFFERED_MSG_COUNT:5}"

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