Ignore SCID changes in Initial Packets

[ripatel-fd]

fd_quic doesn't enforce this rule:

Once a client has received a valid Initial packet from the server, it MUST discard any subsequent packet it receives on that connection with a different Source Connection ID.
...
Any further changes to the Destination Connection ID are only permitted if the values are taken from NEW_CONNECTION_ID frames; if subsequent Initial packets include a different Source Connection ID, they MUST be discarded. This avoids unpredictable outcomes that might otherwise result from stateless processing of multiple Initial packets with different Source Connection IDs.

Should be fixed