MINOR: quic: Add a trace to identify connections which sent Initial packet.

This should help in diagnosing issues revealed by the interop runner which counts
the number of handshakes from the number of Initial packets sent by the server.

Must be backported to 2.7.
diff --git a/src/quic_conn.c b/src/quic_conn.c
index cbead12..36afa58 100644
--- a/src/quic_conn.c
+++ b/src/quic_conn.c
@@ -632,6 +632,11 @@
 					chunk_appendf(&trace_buf, " frm@%p", frm);
 					chunk_frm_appendf(&trace_buf, frm);
 				}
+
+				if (pkt->type == QUIC_PACKET_TYPE_INITIAL) {
+					chunk_appendf(&trace_buf, " with scid");
+					quic_cid_dump(&trace_buf, &qc->scid);
+				}
 			}
 		}