BUG/MINOR: mux-quic: do not access conn after idle list insert
Once a connection is inserted into the server idle/safe tree during
stream detach, it is not accessed anymore by the muxes without
idle_conns_lock protection. This is because the connection could have
been already stolen by a takeover operation.
Adjust QUIC MUX detach implementation to follow the same pattern. Note
that, no bug can occur due to takeover as QUIC does not implement it.
However, prior to this patch, there may still exist race-conditions with
idle connection purging.