Make use of the Order Flag of DATA packet being ignored in live and stream modes

[maxsharabayko]

SRT Data packet header has the "Order Flag" (O). The Order Flag indicates whether the message should be delivered by the receiver in order (1) or not (0).

This flag is not used in stream and live modes.

In live mode, all packet can only be read in order. Therefore this flag is in fact ignored.
However, the default value of this flag is 0 because of the srt_msgctrl_default::inorder = false.

• Change the default value of srt_msgctrl_default::inorder to true?

This flag can potentially be reused in the future.
For example, for packets that are not allowed to be dropped (as suggested by @ethouris). Although the receiver will never know until it receives it.

• Can the Order Flag be reused in live and stream transmission modes and how?

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+- SRT Header +-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|                    Packet Sequence Number                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |P P|O|K K|R|                   Message Number                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination Socket ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                            Payload                            +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

[ethouris]

What I think should be done about it:

1. Make no use of it.

Leave it as is as we don't have any idea how to reused this flat, and we might find something useful in the future.

2. Leave the default false.

This is because this is the current default and as such is used by everyone worldwide. Even though this applies also to a file/message mode, in which case it's not exactly wanted. This is actually the consequence of default live mode. The only solution for that I imagine would be to create an alternative "default" msgctrl object that would have this flag set to true, but I don't know how much it makes sense.