[BUG] Initially lost packets may be ignored in group message mode

[gou4shi1]

Describe the bug
need_notify_loss in CUDT::processData() will be set to false for the first received packet (because the group recv state is transiting to RUNNING).

To Reproduce

1. Callers broadcast inorder messages to receivers.
2. The first packet is dropped by iptable.
3. The following packets are received and the receiver ack with largest_received_seq+1 since the first lost packet is ignored by !need_notify_loss.
4. Group::recv() fail to read a inorder message since the first packet is lost forever.

Expected behavior
I know that if TLPKTDROP is enabled, it should be fine, but in message mode, all lost packets with TTL=-1 should not be ignored.

Desktop:

• OS: Linux
• SRT Version / commit ID: 8518558 with [core] drop packets in the new recv buffer by group recv base #2207 [core] refactor Group::recv() base on new rcv buffer to support message mode #2218 patched

[gou4shi1]

It seems to be introduced by #1448 but I don't know what exactly "runaway train effect" means, is need_notify_loss mainly for backup links?