In 3.4 we introduced TransportLayerLegacy which wrapped up the legacy Listener/MessagePort classes behind a new abstraction. In 3.4, the log message
NETWORK [thread1] connection accepted from 188.8.131.52:34397 #15 (6 connections now open) comes from here:
This gets passed off to TransportLayerLegacy as an AbstractMessagingPort which gets stored in a transport:::Session, which has it's own ID created here:
A thread is started to service the connection, and the first thing it does is set the thread name here:
This is what causes log messages to have their "conn" prefixes like this
2018-04-12T22:42:17.197-0400 I - [conn15] end connection 184.108.40.206:6087 (9 connections now open)
Because the "accepted" log message and the thread name are coming from different ID numbers, it's possible that the "conn15" log messages are not related to the number in the "connection accepted from" log messages.